Hi,
Is there any newly installed software? You might have missing/corrupt files or due to virus infection.
When did this issue first occur? If recently, system restore could be under consideration. However, if the system files were infected prior to the date you would like to restore to, system restore won't help. I suggest run a full scan using the antivirus software installed on the system.
In addition, you can get a free PC Safety scan via windows onecare and clean all corrupt registry to check the result.
http://onecare.live.com/site/en-us/default.htm