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.