There are two things to check out :-

1. Make sure the error message is shown on the screen by turning off 'Auto restart', i.e. right-click My Computer, Properties, Advanced Tab then Start Up/Recovery, Settings at the bottom and untick 'Auto Restart'. Next time you get the error your computer will display a blue screen with a STOP error message. Make a note of the number (format 0x00000001) and look it up here http://www.aumha.org/a/stop.php or post it back here on the forum. You will have to hold in the power switch to switch off the machine before turning on and rebooting.

2. Make a note of the time you next have the error and look in Admin Tools, Event Viewer for a clue to the problem.