Any other particulars.
There are a myriad of possibilities - drivers, av/antispyware/security products, software updates and of course, hardware problems
can cause that error. As mentioned, it would be wise to back up all of your vital data right away, if you haven't done so already.
Then start troubleshooting.
Have you updated any drivers lately? Check and see if Windows Updates has done any updates recently. Have you added hardware, changed security products, or added new software?
Any major devices not functioning properly?
have you run Check Disk? If not, it is a good idea, and may help.
Use "BlueScreenView" to capture BSOD info, and post it here.
"BlueScreenView" is an excellent tool for posting Blue Screen Error Information
For the BSOD - "BlueScreenView" will scan all your minidump files created during BSOD crashes, and display the
information about all crashes.
Mircorosft.com - "How to fix BlueScreen (STOP) errors that cause Windows Vista to shut down or restart unexpectedly"
How to fix BlueScreen (STOP) errors that cause Windows Vista to shut down or restart unexpectedly
A great site for troubleshooting BSOD under Vista - Troubleshooting Vista Blue Screen and Stop Errors
Best of Luck.