Windows – What is causing hard drive grinding?

My windows (XP) hard drive grinds a lot and it negatively affects the performance of other applications.

I have plenty of RAM and have already verified that this hard drive activity is not page file swap activity.  I have also already looked in task manager to see if only one or two processes are active but I have found no smoking gun.

What tools can I use to directly determine what application or feature of the OS is causing the hard drive activity?

Solution:

Process Explorer.  Make sure you show the columns that contain the I/O activity.

Sometimes you may see a process running multiple services that causes the activity.  In that case, it requires a bit of guesswork.