I had that exact same problem too. Mine came up when I updated my catalyst control center to include the 'overdrive' crap they have.
I had the same pentium 4, 1 gig ram, a stock dell mobo, and a Radeon x800 XT PE.
What i ended up having to do, was uninstall Catalyst control center, and install: 'ATI Tray Tools' by Ray Adams. This program is specifically for radeon cards, and lets you clock it however you want, instead of using the 'dynamic clocking' or whatever the overdrive thing in catalyst is.
try doing that, and make sure your drivers are updated properly.