So I was just messing around with the disk management utility, and I clicked "mark as active partition" on a random partition. Then, I try to click "mark as active" on my system partition ( which I beleive was the active one before), but the button is greyed out.

My understanding is that if i reboot now, win will try to boot from my random ( H: ) partition and not my C: partition -- which would be a problem. I tried to use the diskpart command line utility to set the C: partition as active, and it said it succeeded, but when I looked in disk management, H: was still the active one. Help!

There's a couple of things you can try.

I tried to use the diskpart command line utility to set the C: partition as active, and it said it succeeded

Try booting. The change may have "taken" if it reported so.

If that doesn't work, you can try booting from your XP CD and entering the repair console. Once there, run the bootcfg /rebuild command. This should find the Windows installation and correct things. A discussion about the Bootcfg command and its uses

There is also - How to recover from a corrupted registry that prevents Windows XP from starting; How to recover from a corrupted registry that prevents Windows XP from starting

If this doesn't work, you can try a Repair Install; How to perform an in-place upgrade (reinstallation) of Windows XP

Post back with your result.

Well, I was keeping my computer on cause I didn't want to reboot, but my power went out and so now I just turned my computer on and it worked fine. H: is still marked as active in the GUI disk management utility, but whatever... if it ain't broke don't fix. Thanks anyways!