take a note of the time and check your Event Viewer to see if there are any clues.

The Event Viewer can be accessed by going to Start > Run, type in eventvwr.msc then click OK. In the Event Viewer window look at the events under Applications and System for any event at the same time as the lock up. They may have either an exclamation mark for a warning or a cross for an error. If you see any, highlight the entry, right click it and select Properties for more information.

Are there any clues you can give to what you were doing just prior to the lock up, eg using a specific program or application, surfing the internet, or downloading files?

Mark