Start pulling sticks of RAM one at a time. I've had several computers come to me with a corruption of Fastfat.sys for repairs, and every time but once it was cured by pulling the offending DIMM.
The one time it wasn't was due to an upgrade from WindowsME. If all your sticks are kosher, reboot with the XP install disk in the drive and run the installation repair portion of the setup. Should fix 'er up good.