First of all make sure the error message is displayed on the screen by stopping the computer from automatically restarting when an error is encountered by right-clicking My Computer > Properties > Advanced Tab then click Settings at the bottom alongside Startup and Recovery then in System Failure untick ‘Auto Restart’.

Now, next time you have a BSOD, the STOP error will be displayed on the screen. Make a note of the error number (the format is 0x00000aaa, where aaa is the hexadecimal error number, e.g. 05A) and look for an explanation of your error number as shown in the left pane here http://msdn2.microsoft.com/en-gb/library/ms795508.aspx