First and formost,
Antivirus and antispyware scan.
Do this at least 2-3 times until you are most certainly assured the machine is clean. More times then not you have a maleware attempting to take control of an application that is causing the BSD.
Next, check with www.windowsupdate.com
to make sure you have the most recent drivers for your machine.
Next, check Video, Sound, and peripheral drivers to make sure you are up to date on those.
Next, check out what processes you have running on your machine to see if anything looks out of the ordinary. Start killing off non essential processes.
After each one of these make sure to note the way your system is acting so you can try and replicate what you are doing right before it crashes. Also try and MAKE it crash. Once you can duplicate the crash process you can more easily fix it.