Antivirus, Trojans, Spybots, and other Malware
I run Linux, so I rarely have to deal with this stuff, but sometimes I am called on to help a friend with something suspicious. There are services online that will let you upload suspicious files for online scanning. This way I don't have to deal with installing software and it doesn't matter that I'm using Linux.
If I do install software then I usually go with Spybot Search & Destroy. For antivirus software I install Avira or Microsoft Security Essentials. I've been told that Panda Security Cloud Antivirus is good too, but I have not tested it. These are all free. Trend Micro HijackThis can also be handy for creating reports about the state of a system. It is useful when investigating what might be wrong. It does not actually find or remove bad software.
List of Antivirus software
Jotti This is a meta scanner. It compares our files against many virus scanners run on its server. You upload file; it scans and reports. Dr.Web Online Virus Check You upload file; it scans and reports. Virustotal online file scanner. You upload file; it scans and reports. BitDefender QuickScan Easy to install full system scanner. You have to agree to allow it to install a browser plugin. Anubis: Analyzing Unknown Binaries This site will actually run your executable and tell you what it does to the registry, filesystem, memory, etc. You upload file; it scans and reports. ISEC Lab for more cool projects from the same people that provide the Anubis service. Malware Help This is a list of online virus scanners (rather like my list here).