My experience has been that blue screens such as this are most often caused by buggy drivers, but figuring out which one requires analyzing the dump.  If you can catch the blue screen, it may also indicate which driver is involved.

If I had to guess, it would most likely be the Video card driver.  I'd check for the most recent drivers available, and load those and see if the blue screen issue goes away.

Also - I believe that Vista does have a memory diagnostic tool.  Memtest is the name, if it serves me right.  If you suspect physical hardware issues, that might be one route to take.