FREE Diagnostic Tools
Malwarebytes Anti-Malware 1.33 http://www.download.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html?tag=contentBody;mostPopTwoColWrap&cdlPid=10997763
Though a relatively new player in this area it has already proven to be an excellent tool to keep harmful malware off of your PC. It is a product we routinely use to clean compromised PCs.
Ad-Aware Anniversary Edition 8.0 http://www.download.com/Ad-Aware-Anniversary-Edition/3000-8022_4-10045910.html?tag=contentBody;mostPopTwoColWrap&cdlPid=10998841
The most popular in terms of number of users.
System Information Wizard (SIW)
Gabriel Topala's application that gives a full, in-depth picture of your entire system. Key information to have in the event of problems.
Trend Micro HiJackThis 2.0.2
A very powerful tool in the hand of an experienced user to clean up seriously infected machines. It assumes that the user is familiar with what is being removed so use with extreme caution.