Ad-aware:

This is a great utility that is able to take a look at your hard drive. It can detect anything suspicious on the computer and gives a report, then you have the option of deleting those entrys, it can also keep a copy that can be restored if any thing goes wrong on your computer.

Spybot-search and destroy:

This is a very clever tool which will allow you to find entrys that may have been put on your computer without your knowledge, and cause a security problem infringing on your privacy. Its Function is similar to that of Ad-aware but is possibly more comprehensive.

Ants:

Ants is a program for detecting and removing trojans, and worms, and is available free.

Eraserlite:

Eraserlite rids your computer of temporary internet files, your history of what sites you accessed, and cookies, etc.