The most common cause of this behavior is a virus. Posting a log for the Pros in that section would at least eliminate this possibility.
Otherwise: Have you tried a System Restore? You can also try to troubleshoot with
msconfig
http://support.microsoft.com/default...;EN-US;q310560
Selectively remove your start up items.
Also view your Error Log via:
[1] Start Menu
[2] Help and Support
[3] Pick a Task
[4] Tools
[5] Advanced System Information
[6] View the Error Log