Justif the chassis has any "warranty void if removed" stickers, you're should not break them, or you may lose your warranty.

Just make sure the computer is unplugged from the mains and left for about 30-60 seconds to discharge any residual power from the PSU, and remove any static prone clothing and jewelry before messing with the insides of your PC.

Just gently loosen the lock pins both both ends of the memory slots, and remove the ram. Blow gently on the sockets to remove any dust particles, and then re-seat the memory modules without using force. Clip the lock pins back into place.

If this does not do the trick, then I'd suggest a re-load of Windows, but you said you've already tried that with XP.