Click Start, My Computer, right click on "Local Disk C", then click on Properties. Under the General Tab, at the bottom, is the box ticked that says allow indexing, or something to that effect?
If it isn't, then your AV or antispyware could be doing a scan.
Other than that, you could have some spyware, and you need to get rid of it.
It is most liketly your Antivirus/Spyware suite scanning in the background. It could possibly be some P2P software such as Limewire depending if you have it installed.
