I've been experiencing some bluescreens recently with Vista Ultimate x64.  The bccode I'm receiving is 50.  After a bit of research, I found that sometimes memory is the cause of this code. 

In the past I did have a bad memory stick that caused my workstation to bluescreen quite freqently.  A third party memory tester from PC Magazine helped determine which stick was bad and after replacement, no more bluescreen.  Sweet!

So to rule this on Vista, I found a kewl built in utility that will test your memory after during the startup phase.  Simply click Start | Run | and enter MDSched.exe.  Follow the prompts, restart, and watch the test proceed. 

In my case, the memory was good.  It's probably a bad driver I've installed so the search will continue.