Please follow these steps so next time you have a bluescreen we can get the code in it to identify your issue. To make sure you can read the blue screen (as in if you see the blue screen for a second or two and your PC auto-reboots and you can't read it ... then follow the instructions below to disable auto reboot so you can write down the code).
You should turn off the BSOD auto-reboot so next time you can see it and write down the error code.
(Note: I am assuming you are running windows XP. If you are not re-post and tell me you are not running windows XP. If you are please follow the instructions below.)
To do this ...
1) Right click on the "My computer" icon in the desktop...
2) Go to properties
3) Go to "Advanced"
4) Go to "Startup and Recovery" and hit the "settings" button.
5) Now where it says in the "System failue" box un-tick the box that says "Automatically re-start".
Now you just have to hope you will have another BSOD (or maybe never have it again) ... but if you do we will need the code from it.

If you get a screen like this the code we will need is after the "*** STOP:" after the bottom (so its STOP: 0x00000050 and so on and so on.