how probable is a failure of two memory modules?

Title sais it all... an old Dell laptop, both modules in all possible combinations modules/slots show errors (memtest86+). Dell service said they'd send me two other modules but I'm quite sure it's the board.

Uhm, just check it with the two new modules and if it still doesn't work burn the Dell HQ down? =/

I had two out of four modules fail on my Dell XPS recently. It gave me some weird beeps. Replaced them to fix it. Just make sure you get the exact type of memory that is compatible with your motherboard or it won't boot.