Another good tip is to download FileMon from sysinternals (just google it, it's very popular for many techs.) It hooks the filesystem and tells you which program is touching what files, in real-time. If you've got some process that is repeatedly banging on the hard drive for some asinine files, it's probably sucking not only disk resources but others as well. Terminate whatever services or processes are responsible.
On the same note, you will often see svchost.exe responsible for all kinds of activity but you know you can't terminate it, since it hosts services. I believe Process Explorer (also sysinternals) or another 3rd party one (called Security Task Manager, not free) can help you identify which services a given svchost.exe is responsible for. It won't boil it down to one service, but with a little bit of research on the few services a particular one is running, and the files it is accessing, will give you an idea of which service to stop.
I wrote this a while back:
How To Build a Windows XP Chopper
http://knepfler.com/chopper (never fully completed but very thorough)
Which is basically an experiment in how many services can be stopped and still have a functioning system. I think I got it down to one service.
It wasn't very practical as many things were broken, but you can probably get it down to less than 10 pretty easily and cut all kinds of unnecessary memory use and disk access.