Memtest question

[cmdrdredd]

I didn't exactly know where to put this so here it is. I have 2GB of Corsair XMS2 6400 running at 800Mhz at 5-5-5-12 (rated timings) I have no issues in windows, my system is prime stable at 400Mhz FSB, 3.2Ghz CPU (e6400 w/ P5B Deluxe). Then I tried memtest and it errored. I'm sure my PSU can be an issue as well and will replace it soon anyhow, but I was wondering. If windows is stable, how likely is it that memtest is incorrect in telling me that my memory is bad?