Have you ever scanned your pc for viruses or ad/spyware? I would suggest you download Spybot S/D from
http://www.safer-networking.org/en/index.html and scan your computer for spyware. Once you install spybot, be sure to use the option in the program to update files.
Also, if you do not use antivirus, you can go to
http://www.trendmicro.com/en/home/us/personal.htm and use the Housecall online virus scanner.
If these come back clean or do not solve your problem, let us know.
