Have you installed any software recently? Added any hardware? Made any changes to your computer at all, including any Windows updates? If so, that's the first place to look for the culprit.
If you haven't made any changes recently, the next most common culprit is spyware. If you haven't scanned for and removed spyware recently, ownload, install,
immediately update and then run
Spybot and/or
Ad-Aware. Read the help files to familiarize yourself with how they work. I recommend you use them both since each tends to find things the other misses. Let them clean up any spyware found.
Then, to be doubly sure, download and run
CWShredder.