If you are referring to the hard drive activity light, then something is causing hard drive read/writes.

Have you added or changed your anti-virus or ant-spyware programs?
You could have an overactive scanning for malware. (check settings and logs for activity)

It could be another corrupted program or malware causing hard drive activity.
(shutdown all background programs or start in safe mode)

Could be your hard drive is failing. Hard drives constatnly correct themselves, when a bad sector of the drive is detected, the drives move the data and marks that sector as bad. If your drive is failing, sometimes it can cause this symptom. (run a hard drive diagnostic program)

Check those things and let us know.