|Check cooling is OK; all fans etc. are running and vents clear - although I doubt this a cooling issue...|
This may be a RAM isue; one or more sticks starting to fail...
Reduce to a single stick - min of 128Meg is nice for W2K; try each stick in turn. If some work OK and others not so... replace/renew those that produce the problems.
Also run a full chkdsk routine; allow it to find and fix problems. If it reports it has done so and gives an indication of how much/many - and it's more than just one or two... - seriously consider replacing the drive; but only after further and more specific tests on the drive (using test utils from the drive's website).