Lets try to get the set up straight...
You have 2 hard drives right now, one with WinXP that you use for audio software, and the other (HD?) for another OS? And you upgraded the one hard drive that you use for your other OS with a new version of Windows XP SP2?

I guess the questions I have is, obviously, why? If you have Windows XP on both drives, why not just combine them? Is this an office PC?
I don't think there is really any way any of us are going to be able to help you. We have no way of knowing what you changed in your registry. :-\ I would say to try to reverse everything you did in the registry so you can at least get to the point you were at. Then back up everything you can/want to and just do a fresh install of the whole system.

I can suggest a little more stable way of doing this dual boot though. My friend has this set up, and I've been trying to get around to doing it on my PC. He has a removeable hard drive bay in his computer, and he just puts in whichever hard drive he wants to use. He has one with Windows XP for games, then one with Linux for everything else. This would be how I want to set up my computer some day. You can buy the bay, and 2 cages for relatively cheap.