Okay, OT. Here's the basic run-down on what you want to do. Let me know if you get to a step and it doesn't make sense, or you don't see it. I don't have XP in front of me, so I'm having to do this from memory. =)
First thing we want to do is download the latest drivers for your card, which I'm betting you already have done.
Just put them on your Desktop or something so you can get to them easily later.
Next, we need to uninstall any software they may have installed. Go to Start>Run, type appwiz.cpl and hit enter. That should launch the add-remove programs window. Look for anything that says ATI. It'll probably be something like ATI Catalyst, or something like that. It's been a really long time since I had an ATI card, so I don't know for sure.
Once the software is gone, Start->Run, type devmgmt.msc and you should get the Device Manager window. Near the top of the list, hit the plus symbol next to Display Adapters and drop that down. Double-click the video card (if you see more than one here, let me know), that should pop up the properties for the ATI card, go to the Driver tab, and select Uninstall down at the bottom. Just walk through this and choose the obvious options to uninstall that card. If you have any questions, feel free to stop and post and wait for an answer.
Now, let's disable anything running in the background on boot. Start->Run, type msconfig and hit enter. This starts the system configuration utility. Go to the Startup tab (probably last tab on the window). You'll have to use a little thought here, but you're going to go down through the list and uncheck everything in it. Anything that's specifically required to allow you to work on the computer, leave. That means that if printer software shows up, you'll get rid of it. If there's software there for your mouse, then you might just leave that. (Later when you reboot the computer, you're going to get a message about system settings being customized or something like that. Just check the box that says "do not show me this again" and hit okay.)
Okay, now we've done that, we need to boot into Safe Mode real quick. To do this, Start-> Shutdown->Restart. As it comes up, there'll be a point where the screen goes black just before the Windows splashscreen comes up. When you get there, start tapping the F8 key. You should get a menu and be able to arrow up to Safe Mode. Hit Enter and let the computer boot. It'll come up in a really run-down looking form of Windows (jokes about the true nature of Windows comes to mind, but I need to stay on topic!). You'll have to log in (as administrator if it offers it as an alternative choice. Once in, follow the directions above to get to Device Manager, and then to your Display Adapters. Right-click and delete/uninstall all the video cards listed there.
Close everything, reboot, and when you come back up, run the install script fot the files you downloaded. That should install your video card and then ask you to reboot, go ahead.
When you're back up again, try the game.
If it's works, awesome. Just go back to msconfig (as above), but this time, you'll be re-checking the stuff you want loading. If you see something you know you don't want, it's okay to leave it unchecked. (remember on the next reboot, you'll get that message again because you've made a change.)
I know it seems a little long, but it's actually pretty straight forward. I don't think you'll have any trouble with it. Good luck! PM me if you have problems, and I'll get you my phone number so you can call me.