Free Spyware Tools
Disclaimer: I did not write any of the tools listed here. They are all available free of charge on the Internet but I have included them here for your convenience.
HiJackThis
Probably the most powerful free spyware tool I have ever used. It scans the windows registry for any possible spyware risks and reports them all. You have to be careful it reports both the spyware and legitimate programs that start up. As well as running processes. Works with Windows XP/2000/Me/98/95/NT
LSPfix
Fixes the things HiJackThis can't fix. LSP is related to windows ip stack and sometimes spyware adds its own entries into this so that windows ip stack gets cluttered with flagrent LSP entries. newdotnet is a good example of spyware that does this. If you are having trouble accessing the Internet after removing spyware you need to run this tool.
AVG Anti-Spyware Free
Now the best free anti-virus company has made a free Anti-Spyware product. This product is %100 functional for 30 days and after that the resident shield, which block spyware as it is being installed, no longer functions. I use it as a final scan after I run Hijackthis and other to make sure that all the spyware is completely gone. It takes a while to do a quick scan, around 30 mins but it gets almost all the spyware. Unfortunately it only works in Windows 2000 and later.
Windows Defender
One of the best the best spyware scanner and remover I have ever used. It is free but still in beta and surprise, Microsoft didn't actually make it but purchased it from Giant Software. It is easy to use, automatically updates and automatically scans. It is good at detecting spyware that tries to install itself. The number of pop-ups and security warnings it displays can be annoying and confusing to new users but it is a very good program. Windows Defender is the newer version but has the disadvantage of only running on windows xp sp2, and it can be a pain to install because windows has to be validated which is hard to do if spyware is keeping Internet explorer from working.
AVG Free Virus Scanner
AVG is a free virus scanner for personal use. It the best Virus scanner I have ever used. It is fast, it does not have the bloat that Norton and McAfee have. It can detect viruses on the fly and remove them. It automatically updates, checks for viruses in realtime and can do scheduled scans. I usually install this program and it runs while I do a spyware scan with a different program and AVG will find viruses while the other scanner scans your hard drive. Works in Windows 98/Me/2000/XP.
Spybot Search and Destroy
Spybot is a good scanner when you can't install Microsoft Antispyware. It can even get rid of things that Microsoft's scanner can't. It also has many features that help prevent spyware from being installed. Its greatest strength is that it works on all versions of windows, even 9X.
Clam Anti-virus
Clam Anti-virus is a GPL virus scanner. It does a better job of detecting viruses than AVG and its database is updated more regularly. Unfortunately, it can just detect and report viruses. It is up to you to remove the Virus. It is useful for scanning downloaded files and viruses before you run them. It runs in Windows as well as *nix. It is more commonly used by servers to filter mail that contains viruses.