A couple of years ago most folks relied on SpyBot Search and Destroy and Ad-Aware for spyware protection. Alas, spyware has evolved so quickly that these once outstanding products are no longer up to the task of providing primary protection, though they remain useful as secondary, on-demand scanners.
The new generation of malware requires a new generation of products. Such products need to provide stronger active protection and broader spectrum detection. The best anti-spyware programs (WebRoot SpySweeper, Spyware Doctor and CounterSpy) are all commercial products, but there are three free products that I can recommend that provide real time protection.
The first is a special cut-down version of Spyware Doctor that's available as part of Google Pack which is a bundle of free software offered by Google. Called "Spyware Doctor Starter Edition”, the cut-down version lacks the full array of real-time-protection monitors possessed by its commercial big brother, but still offers some active protection along with full scan and remove capabilities, scheduled scans, and free signature file updates. And on the subject of signature files it should be noted that the signature files in Spyware Doctor Starter Edition are neither as large nor as up-to-date as those in its commercial big brother.
Your second choice is Microsoft's Windows Defender. Defender is the latest reincarnation of the excellent Giant Anti-Spyware product that Microsoft purchased late in 2004. Based on my tests, Windows Defender is not as effective as its immediate predecessor, but still has solid protective capability. I tested it on several drive-by download sites. Its multiple real-time monitors provided a reasonable (though by no means watertight) defense. It appears to be a little vulnerable to polymorphic malware in particular, and for this reason I suggest it should be used in combination with regular on-demand scans from the free AVG Anti-Spyware. My other reservation about Windows Defender is that it consumes quite a lot of your processing power. If you have a modern PC this should not be a problem. However, older machines will definitely suffer a performance hit.
Note that you need a legal version of Windows XP SP2 to run this program. I've been told that cracked versions of Defender will run on any XP SP2 PC and are currently circulating on the P2P networks, but I'd approach those with caution. The idea of a cracked security program must be considered an oxymoron.
A third choice is Spyware Terminator. Unlike Spyware Doctor or Windows Defender, it works with all versions of Windows, so it's the stand-out choice for Windows 9x users. It's no slouch either. Like Windows Defender, it has strong active protection. Indeed, with it’s built in HIPS system that warns you of any unrecognized intruders, it has stronger protection against unknown threats than the Microsoft product. This was confirmed on some tests I ran on drive-by download sites where Spyware Terminator proved to be impregnable.
Spyware Terminator has its own spyware detection engine, and gives you the option of using a second engine based on the Open Source ClamWin anti-virus program. ClamAV is not the most effective AV scanner on the market, but it's certainly competent and the additional protection can only be a plus.
On the downside, Spyware Terminator is slow to scan and can slow down your PC a tad, though not nearly as much as Windows Defender. I've also heard reports that support via the free forum is poor.
Choosing between Spyware Doctor Starter Edition, Windows Defender, and Spyware Terminator is not easy. The full commercial version of Spyware Doctor has outstanding detection, but the reduced signature file size and weak active protection in the free starter edition is a concern. Defender is heavy on resources but has reasonable active protection. Its detection, however, is mediocre.
As of today, I think Spyware Terminator has the edge as the best balanced product of the three, but it's difficult to see how free products like this can remain viable, particularly in a high support product class such as anti-spyware.
However, if you use an anti-virus product with good, active protection, such as the free version of AntiVir, you may well be tempted to go with Spyware Doctor. It's a product which offers reasonably good detection and excellent removal capabilities, and is backed by regular and reliable signature file updates. Note: Spyware Doctor Starter Edition can be obtained via the Google Pack, but you can get it as a stand-alone download.
If all you are looking for is a tool to scan for and remove spyware/adware, you should definitely consider SUPERantispyware. Though the free version provides only on demand scans, it has a growing reputation as the one to use for detection and removal of hard to kill strains. It is also one of the few new products to be added to to the well known Spywarewarrior trustworthy antispyware list .
Another product in the same mold (the free version provides only on demand scanning) is Malwarebytes' Anti-Malware. Though very new, it has a good reputation as the organization behind it is well known for being a strong supporter of the fight against spyware and has in the past released other excellent freeware tools like RogueRemover Free.
Google Pack
Website: http://pack.google.com/intl/en/pack_installer.html?hl=en&gl=us
Download link: http://pack.google.com/intl/en/pack_installer.html?hl=en&gl=us
Author: Google
Current version: 2.2.940.34809
Version date: August 28, 2007
License: Freeware
Download File size: 845KB
Operating Systems Supported: Windows XP or Vista
Additional Software Required: Microsoft Internet Explorer 6.0 or later or Mozilla Firefox 1.0 or later
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: Yes
Other relevant information: None
Microsoft Windows Defender
Website: http://www.microsoft.com/downloads/details.aspx?FamilyID=435bfce7-da2b-4a6a-afa4-f7f14e605a0d
Download link: http://download.microsoft.com/download/e/d/0/ed099d5e-dc60-4740-8747-1c72f053b800/WindowsDefender.msi
Author: Microsoft
Current version: 1593
Version date: May 23, 2007
License: Freeware
Download File size: 4.9MB
Operating Systems Supported: Windows Server 2003 sp1 & sp2/Windows XP sp2
Additional Software Required: None
64 Bit Capable: Yes
Portable Version Available: No
Additional Software Required: Microsoft Internet Explorer 6.0 or later
Non-English languages supported: Yes
Other relevant information: Genuine Microsoft Windows Validation Required
Spyware Terminator
Website: http://www.spywareterminator.com
Download link: http://www.spywareterminator.com/download/download.aspx
Author: Crawler, LLC
Current version: 2.0.1.224
Version date: September 27, 2007
License: Freeware
Download File size: 8.51MB
Operating Systems Supported: 98/Me/NT/2000/XP/2003 Server/Vista
Additional Software Required: None
64 Bit Capable: Yes
Portable Version Available: No
Non-English languages supported: Yes
Other relevant information: At least 10 MB of free hard drive space/ 2.1 beta released 1/12/2008
Spyware Doctor Starter Edition
Website: http://www.pctools.com/spyware-doctor/google_pack/
Download link: http://www.download.com/Spyware-Doctor-Starter-Edition/3000-8022_4-10704508.html
Author: PC Tools
Current version: 5.1.0.273
Version date: November 12, 2007
License: Freeware
Download File size: 14.72MB
Operating Systems Supported: Windows 2000/2003/XP/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: Yes
Other relevant information: None