I'm no expert, but it sounds like some sort of virus to me. I suggest you try
this free antivirus scan thing and download 'AVG' from Grisoft
here - it's free. You may also want to download
SwatIT which is a trojan removing program. For further info about Trojans see
#NoHack's website.
Make sure you also have no scripts loaded which could cause this to happen.
G'luck!
Regards,
