++Fastolfe for the previous post.

If you have multiple websites running on the NT server- I.e. you are a webhosting company or whatever, you could also try the following:

Make sure that all webstes applications are running in their own memory space/isolated process.
This will allow you to see the resources for each website in task manager. - There will be lots of MTX processes. (1 mtx for each website I think). You will be able to see which one is consuming all the resources.
You won't know which MTX relates to which website, so download the freeware HandleEx from SysInternals
HandleEx will give information pointing to which IIS website is causing the problems.

You will then have narrowed things down a lot. Having websites run in their own memory space is a good idea because it means that if one website crashes, the others will not be affected.