Try 'disable in current hardware profile' in Device Manager for the onboard video. That way there's enough of it for the new hardware wizard to not want to load the drivers, and hopefully less enough of it to not interfere with the Radeon Card.
I can't tell about the possibility your motherboard has to disable the onboard video in the BIOS. Tell the motherboard details so maybe somebody else knows more than is mentioned in the manual (but that isn't likely, I'm afraid).
And what's the problem with 'The driver software does not seem XP Pro compatible'? How do you know? What's the effect? Looked on the manufacturers site for info of a better suited driver? You don't tell the manufacturer of the video card either, so we can't help you with that either.
I just purchased a PCI 64mb Radeon 7000 Video Card with DVI output. After installing the hardware, and pluging in the monitor, I get nothing. No monitor at all. I have tried unistalling the onboard video drivers and reinstall the Radeon card, but when I reboot, the computer keeps finding the onboard video and trys to reinstall it. There is no option in BIOS to disable the onboard video as far as I can tell. Have also referred the the motherboard book. The driver software does not seem XP Pro compatible.