you prob have 2 maybe 4 sticks in? I had this same issue and found that it was my motherboard didnt like 4 sticks. After RMA im running perfect, though my mobo did work perfect with 2 sticks. So my advice if you havent tried, try 1 stick at a time and/or 2 sticks at a time.
I believe i got memtest errors though back when i had issues. Vista is very picky with the ram, prob cause it actually uses it. I would say try 1 stick, and if you can a different stick. If that doesnt work it might be your mobo.
Also how are you voltages? the 12, 5, and 3.3?