I upgraded the RAM in a friend's old Dell Dimension 3100 with Windows XP. It was really slow because it only had 256mb of RAM installed. I upgraded it with (2) 512MB sticks of DDR2-533 RAM, and after putting the RAM in and turning on the system I notice that the system restarts multiple times and is unable to fully boot. One time I even got a BSOD. I immediately suspect that the memory I installed was defective.

I then ran memtest, one stick at a time in the same memory slot (the bottom slot), and both sticks successfully completed the test without any errors; However when I ran memtest with both sticks of memory at the same time in both memory slots, Memtest was really slow and kept looping back to 0%, and after a while came up with an error that said "unexpected interrupt - Halting," and the system freezes.

At this point I suspect that the motherboard or memory slot on the motherboard is defective, since each RAM stick passes in one slot, but not in both slots.

But, when I put the original RAM back in (a single 256MB module) it passes Memtest in either the bottom or top slot. At this point I am totally stumped and don't know whether the problem is with the memory or the motherboard.

Could it be that the computer is picky with what RAM is inserted even though it is compatible?