It may be in the wrong section, but I'll answer it here.
Sounds like you have a little nasty virus or some spyware lodged happily in your computer. I'm assuming you have a virus checker and some sort of spyware protection already. Update these and run full scans of both and that may well solve the problem. Don't be tempted to download another virus checker and run two at the same time as that is completely counter-productive.
If you have no spyware protection or virus protection, and in this day and age, you'd be mad not to, then download one of the following programs and start there. The first two are virus checkers and are both good, I prefer Avast, but that's a personal choice.
AVG Free
http://avast.download--free.net/
Avast Free
Next make sure you've got some form of spyware checker, again there are several excellent free programs out there and you can have more than one of these, and I would recommend you do as most of them will pick up something that the others have missed. I've never found one spyware product that has picked up everything in one scan:
Windows Defender
Ad Aware - click on the free download link on the right
AVG Spyware
If you haven't already, download and run these three programs and see how you get on - they're all pretty simple to understand. Make sure of you can see an option for full/deep scan that you check the box or choose it. It will take a while, but you'll get there i