Unfortunately issues like that can be a long, slow process of elimination.
First thing I would try is a virus scan at trend micro.
go to www.antivirus.com and use the free online scan there. If you have
trouble finding it try housecall.trendmicro.com.
Try uninstalling Norton and/or McAffee's. Try running Ad-Aware and Spybot
Search & Destroy (use google to find each, or go to downloads.com - just be
sure you get past the "sponsor links" and download the actual product. Each
of those two spyware scanners will sometimes pick up things that the other
misses.

If after all of that you still have the issue (I bet ya won't) uninstall
other applications. AOL is a good next step, they've admitted that they
install spyware (Actually it's AIM that does it, not AOL specifically) called
WildTangent. Uninstall anything related to AOL that says it works
on/fixes/prevents/scans for virii or spyware. Next uninstall other
applications, like Office, though I really doubt you'll get that far and
still have a problem. If something refuses to uninstall, that may be a sign
that it's the culprit. All of the products that I mention above (Norton,
McAffee's, AOL, etc) have utilities available to uninstall them if they get
"stuck". They can be hard to find. I just used Norton's one day last week.
Reply here if you need one and can't find it, I'll help you locate it.

Good Luck!!