Do you have JAVA installed on your system? If it is less than Java 6 Update 26 you have a huge security hole there.. the little orange box in your system tray is annoying, but its your friend also.http://www.oracle.com/technetwork/topics/security/javacpujune2011-313339.html#AppendixJAVA
I'm not sure what antivirus you are using, but please take a moment and perform these tasks on your system for some extra peace of mind (or wipe your system with DBAN which will COMPLETELY erase every sector on your drive, then reinstall everything fresh and restore your backups)...
For quick and dirty rootkit detection you can download GMER and do a preliminary scan..http://www.gmer.net/
If GMER detects things that your antivirus has missed, then either manually hunt these buggers out of your system, or take my next recommendation...
Please boot your system with the CD/DVD/USB drive created from this website, and perform an Offline System Scan, typically capable of detecting some of the more advanced cloaking, rootkit techniques, and rogue JAVA code living on your drive that an Online System Scan will miss.http://connect.microsoft.com/systemsweeper
Finally.. please consider purchasing Malwarebytes as an add on security layer to your existing antivirus solution, it co-exists well with most other antivirus solutions... You can get a free scan and clean from Malwarebytes following (or prior) to the MSSS boot scan.
It will scan and clean your computer for FREE, and if you would like it to actively protect your system its very cheap. Malwarebytes has some sophisticated heuristics detection routines, a dynamic and fully automatic IP blocking system against the bad guys, and I have found that it does an AMAZING job for the money..
Please run GMER one last time... If GMER finds a modified MBR (Master Boot Record) you NEED to clean it manually.
For Windows XP you should be able to boot with your OS CD and pick "Recovery Console" as one of the first options.. Use the FIXMBR command..http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/bootcons_fixmbr.mspx
If you are on Windows 7 or Vista you will want to use the BOOTREC command.. here are some docs on that.http://support.microsoft.com/kb/927392
If this doesn't help you, hopefully it helps someone!