There are several viruses that will do this, and your computer may be
infested by one or more of these.  

1.  Make sure your antivirus software is completely up to date, with
the virus definitions not less than a week old, and do a complete
virus scan of your hard drive.

or

2.  Go to http://housecall.trendmicro.com and do a free online virus
scan of your computer.

or

3.  Go to http://vil.nai.com/vil/stinger and download the free Stinger
virus removal utility and run it to clean the more common of the
current crop of viruses from your computer.

Good luck

Ron Martell     Duncan B.C.    Canada