I'm still betting on infection as the problem. Just because anti-virus and antii-spyware doesn't detect it doesn't mean it's not infected.
If your on a single user machine and your user account as full admin access, I doubt it's a problem with the OS, unless you have changed or installed something RIGHT before it started happening.
As I think I said elsewhere TrueCombat League saw a very simular issue last season with clan [OUT] having 2 or 3 members infected.
They fought the problem for days, and some sort of malware was finally detected BUT nothing would kill it.
I beleive they ended up doing a reformat and reinstall to be able to play online again.
Really though, since this is only affecting you and it only begun happening recently and "all of a sudden" this points toward some sort of malware infection to me.
You COULD try running different anti-virus, anti-spyware stuff. For example if your running Norton, try McAfee or TrendMicro as for spyware try Adaware or PestPatrol.
http://housecall.trendmicro.com/
http://download.mcafee.com/us/eval/evaluate2.asp
http://www.lavasoftusa.com/
http://home.ca.com/
I personally run Symentec Corporate Edition(excellent, much better than any of the home editions), PestPatrol and Adaware.
I hate to recommand you wipe ur harddrives and reinstall, but often it IS the easier way.