Windows 7 - Hard disc activity after screensaver jumps in

On my Windows 7 Ultimate 64Bit machine the harddisc shows a lot of activity after the
screensaver starts.
As soon as I end the screensaver this stops.
Of course it is not possible to check that in task manager (unless I missed some
recording functionality of the task manager).

This only happens after approx 8pm.
I was looking for some planned maintenance stuff or some antivirus things but I cannot find any.

Is there some tool, that records the processes that access the harddisc?
Are there any built in tools for such a task from Windows 7?

Thanks in advance.

Edit: I also checked any indexing functionality but cannot find any.

Hello there ME2g...

What i suspect, (but cannot confirm 100%) is happening,is that as your screensaver starts the system assumes your PC is idle. thus the Superfetch service in your system will work to optimize your system, cataloging your most used programs and applications from your Hard disk and cache them into memory....

Hope that helped!