Check you computer for viruses and spyware.
Update your Anti Virus software with the latest definitions and run a full check.
If you don't have any Spyware installed try these FREE Downloads :
Spyware Blaster : click here
Ad Aware : click here
After downloading use the facility within each program toobtain the latest updates then carry follow the instructions to do a full scan of your computer.
If the above doesn;t work consider using System Restore to a date when you knwo the computer was working properly : Start / All Programs / Accessories / System Tools / System Restore.
All the best,
"Boot-It-Out" click here