First of all, run a defragging program. The built-in one will do the job passably, but I would strongly recommend getting either Perfectdisk or Diskeeper. Definitely worth the money.
Before you run a defragger, check how much empty space you have on your hard drive. If it is 99% full or something like that then fragmentation of files will become a huge and growing issue. Ideally, on a PC one wants upwards to 20% free on the drive, and one should also defrag on a regular basis.
Massive disk activity all the time can also be an indicator of too little memory in the computer. The more programs you open, the worse it gets, so if you have a lot set up to autostart and then run multiple programs too, your PC will swap out content from the electronic memory onto the hard drive.
Combine too little memory with a too-full harddrive that is heavily fragmented and you will be experiencing some pretty hellish harddrive grinding and performance loss.
So:
- check that your drive isn't full or nearly so. If it is, free up space somehow
- run a defragger. Possibly multiple times in a row if you haven't run it before
- check how much memory your computer has and consider an upgrade. There is no such thing as too much memory.
Oh, and before you start out on any of those - check that the hard drive seems to be working right. Do this by choosing File, Run and type "cmd". In the command line window that should pop up, write: chkdsk /r c:
It will give you an option to queue the job to execute it on next reboot. Select yes and then reboot
and be aware that this will take a long time to do, as chkdsk will now scan your entire hard drive surface for errors.
All the above assumes you are running Windows 2000 or later.