maybe you don't know that, on windows vista, if you think you have a problem with your memory, you can test with vista.
Before your computer start loading windows push 2 time ESC key, you arrive on a memory test sotware that is integrated in vista (it looks a bit like memtest).
I tried this with Windows 7, but Microsoft has not integrated it seems
[edit]
this exist too on win 7 but if you push 2 times ESC key the pc reboot, if you want find this, just press F8 before windows boot, and choose repair computer. They are a list with differents options you can use (the memtest are there)