Looks like some Malware/Virus/Program/Service is causing that.
Assuming you are using WinXP, try to boot in Safe Mode by Pressing F8 key when system reboots.
If it works, Check the startup apps / services you are running.
You can do this by typing "msconfig" in Start->Run.
In the Services & Startup tabs, de-select the services/startup apps you find suspicious.
Else, in the General tab, select "Diagnostic Startup".
If it boots normally into Windows, scan using a online scanner:
http://www.kaspersky.com/virusscanner
or any other AV program.
Let me know if any of the above worked.