Seems like a virus. Install an anti virus software and it should fix it. Take a look at Symantec Antivirus. If it doesn't help reinstall OS, that's always the best solution.

This sounds similar to the Blaster Worm that went around a while back. If you still have trouble, let me know I have the removal tool.

Same problem I have also experienced. I have done manual clean up and then loaded with Avast. From past 6 months I did not get any problem.

I'm pretty sure it's a virus or some spyware because I have experienced the same thing a few months back on my old laptop that doesn't not have an Anti Virus software. Bought a Norton Anti Virus right away and problem is fixed.