Upgrade from XP to Vista Business...personally, I prefer clean installs, as they are much more reliable and pretty much eliminate any compatibility problems. It sounds to me that you still have the XP drivers lurking on your system as they were not properly removed and upgraded to the Vista versions before you did the Windows upgrade.
First, are you using an AGP or PCi, PCi Express? Check the documentation or the box that your card came in.
If it's an AGP, then you must
update your motherboard drivers first. Find out the manufacturer of your motherboard and chipset. Make sure you install the latest drivers.
Then remove the Radeon drivers completely following the instructions in the guide or from HERE:http://www.tweakxp.com/article37536.aspx
(it's for XP but should be fine for Vista as well)
If, after you uninstall the ATI driver, Windows may nag you to install the standard VGA device. Click NO (you may have to do this twice). You will then see a note that the device has not been configured properly...you can ignore that.
Download the latest driver from the ATI website that is for Vista compatibility, install and reboot the computer to see if it resolves your problem. If you're installing the entire Catalyst Suite, make sure you have Microsoft .NET Framework 2.0 installed.
If this doesn't solve your problem, then find out if you already have an integrated video card on your board. If this is the case, then you need to boot into the BIOS and find an option that allows you to use an added video card if it exists in the slot instead of the built in one. Save your settings and reboot.