GX100 Graphics Card - Help!!!

Hi, I'm experiencing a problem with my Dell Optiplex GX100 (Yeah, it's kinda old, but it was only 49$, and I needed a "less valulable" computer to mess around with.) Anyway, it came with a Matrox G400 PCI card pre-installed. It came with Windows 98, so I installed Ubuntu 6.06. Everything worked fine until I rebooted. When I got to where I should get the splash screen, I get a black screen with a white line at the top. "Oops..." I thought, and installed XP Home. I also set my machine to use onboard graphics (a old 1999 intel chip). Booted into windows fine. I retried the Ubuntu LiveCD, and that worked too. Then I realised how bad the graphics were. Won't go past 24-bit color, any 3D is either pixelated or grainy, or refuses to display. I set the graphics card back to "auto" in BIOS and rebooted. I got the XP loading screen right up until it should display the login screen, and it froze. I think it's because my computer can't decide which card to use, and I can't disable onboard graphics in BIOS. Help, anyone? Cool Dude