Firstly....
Go to your ad/remove programs dialog. Google everything that you don't reconize that you put there. Remove everything that appears to be spyware. If you have any questions, check a few sites.
Secondly...
Get a Copy of Spybot, Adaware, MS-anti-spyware, Mcafee Stinger and AVG anti-virus. Install/Run these in the order they are presented below.
http://vil.nai.com/vil/stinger/http://free.grisoft.com/doc/http://www.spybot.info/en/index.htmlhttp://www.lavasoft.de/software/adaware/http://www.microsoft.com/athome/security/spyware/software/default.mspxAll of them will remove malware (if present).
After that you should have signifigantly fewer issues.
The First item (singer) will remove most malware.
I used to do this regulary for customers at a small IT company in NoVA. Not the most elegant solution, but it works.
Also run Firefox or Opera as your web browser. They are less commonly targets of spyware/malware.