Sometime ago I had a problem with an old Radeon 3200 PCI graphics card on XP. XP SP3 was not supported, but it worked OK for me. Then all of a sudden the graphics card would more often than not, fail to boot up or crash. I suspected it was an automatic update that had caused some sort of conflict. Not being supported, and unable to resolve the problem - in the end I gave up on the card and replaced it.
Having done a quick google for the 9800 Pro, there are people who have problems with it (besides you) – one suggestion (on those threads) is to re-install whatever version of DirectX you are using.
You might have to replace the card with something more up to date – you can pick up a bargain on ebay (that’s where I got mine).