I take it the grinding means churning/working hard when the computer is idle and not a physical noise caused by a fault in the motor right? If so, then here are my justified guesses:

1. FAULT HDD OR LOST CLUSTERS. Check your hdd's physical condition. Does the access noise sound OK? Does it keep spinning up and down? Run 'chkdsk' on your machine by doing the following:
a. Start -> Run -> type 'cmd'
b. In command prompt, type: chkdsk c: /f
c. It will ask you if you want to run it next time the computer starts up, enter Y. Next time you reboot it will check the drive for errors prior to starting up Windows.

1. YOUR HDD IS FRAGMENTED. Over time installing/uninstalling software, web browser cache, and pagefile reading and writing causes empty sectors all over the place. This means software you install will not be stored in physical order, so hdd works very hard to read and write, hence the "grinding". You should defrag the drive.

If you have a lot of softtware you do not use anymore, it is highly recommended you uninstall it and delete all web browser cache before defragging. You can use the standard Windows defragger (Start -> Programs -> Accessories -> System Tools -> Disc Defragmenter). I use Disk Keeper which I personally think is better but both do the same job.

2. INSUFFICIENT RAM. I used to run 512mb and found it wasn't enough for what I do day-to-day and games. My hard drive would churn like crazy. After adding another stick of 512mb it made a big difference.

3. SCHEDULER. Check your memory-resident software and see if there is anything is set to run either daily or on idle (i.e. antivirus scanner).

4. REINSTALL WINDOWS. I only suggest this if you haven't reinstalled a clean version of Windows for a few years or if any of the above doesn't work. If you install alot of software, it creates clutter, especially in your registry.

It is not advisable to disable VM. It's important for Windows to use your hdd for swapping when you are running multiple programs. Increasing you RAM will help reduce the amount of pagefile writing.

I wouldnt run your computer with your VM disabled. it sounds like you have too many background applications running. e.g some virus scanners scan your computer without asking

When this happens, go to the task manager, and then processes, and see what is running.

Oliver1152 mentioned unscheduled applications, but you might also be seeing normally shecduled virus scans, automated updates... You just need to check the TM and see what is running. You might also look to see what the percentage of use is being shown for the CPU.

Thanks to everyone for your help. I actually fixed my problem inadvertantly. My plan was to double my RAM from 512MB to 1GB, but I needed to check the existing chips to see what kind I had in there. I didn't remember this, but when I bought the PC it had 256MB on board and I added another 256MB later. I took them both out, copied down the info I needed and put everything together. I then realized that the grinding had stopped. I suspect one of two things:
1.) Back when I added the second 256MB chip, I didn't get it seated quite right and for some reason that caused the hard drive to run.
2.) When I re-installed the chips, I changed them from their original positions on the motherboard.
I don't know how or if either scenario would cause that type of problem, but for now at least, everything is cool.

Thanks again to all.

Bryan