That sounds very much like what a machine I had went through long ago - the same progression of symptoms that yours has. Mine also started giving errors in NTOSKRNL.EXE if I remember correctly. I tracked it down to either a problem with the memory or with the motherboard. At the time, I didn't have any way to differentiate which was causing the problem. I finally did a system upgrade and swapped out my mainboard, and at the same time I upgraded the memory, so I never did figure out which.