Any utility that will tell me what program or service is accessing the hard drive?

This is for XP, sp3.

I've just installed XP-sp3 on a new motherboard and I'm in the process
of turning off a lot of garbage services and in general optimizing it.

I've turned off indexing service as well as the cache-thing that's
supposed to make it boot-up faster. The system does auto-login as the
default user (with admin privledges) just the way I like it. So at
power-up within about a minute I have the desktop up in classic mode.

I notice that if I do nothing - just let it start up and sit there, I'll
see the hard-drive light pulse on about once per second - and it will
just to this indefinately.

It bugs my ass and I'd like to know what's goosing the drive like this.

Is there any utility or program that can tell me what's accessing the
drive?