EXTREMELY SLOW OS install/performance on Dell 8200

I am experiencing a first, for me anyway. I am working on a Dell Dimention 8200, attempting to reinstall WinXP (tried Home and Pro) on a new WD 250 gig hard drive. It takes HOURS, I have let it run over night, and it still is loading. It doesn’t really “hang”, in that the progress bar is still moving. I have swapped all cards and drives; I finally got XP Home installed on a spare 40gig, but it took 10 hours. After the install, the CPU Usage in Taskman wil hit and peg at 100% just buy moving a Window around (i.e. LIGHT duty tasks). This unit has RDRAM (yuk, drag). I borrowed 2 sticks to see if swapping them out will help. (will do this tonite) Anyone experienced this before?