How frequently does this happen?
What is the exact error message?
Is the error message always the same?
What are you doing when this occurs?
Make and model of computer?
How old is the computer and when was it last cleaned of cobwebs internally?

The blue screen that shows up happens so fast I can't read the whole thing before it shuts down. But from what I can see of it, it looks like the same message. Sometimes Im not doing anything on the computer when it happens, once yesterday when I was fixing to read my email it happened, another time, I had just clicked on internet explorer.
Its a toshiba laptop intel Celeron, and the cob webs were cleaned out about a month ago.
I ran a tuneup on it yesterday and a full virus scan, and nothing showed up, so I don't know what the deal is with it.

Ah it's a laptop. Are you resting the computer on your lap when using it? Or perhaps a table with a thicker fabric tablecloth. Maybe inadvertently you are covering the vent. This sounds like it might be heat related but as it was cleaned internally recently then I wonder about the vents on your lap.

Perhaps the fan has quit. Can you feel the warm air being discharged or does the fan sounds seem the same as before these issues came up?

Please do the steps below that will enable you to read the EXACT blue screen error next time the pc blue screens, post the error here, a blue screen error can be related to many things this will only be the start of finding the exact cause.

1.Right-click My Computer, and then click Properties.

2.Click the Advanced tab.

3.Under Startup and Recovery, click Settings to open the Startup and Recovery dialog box.

4.Clear the Automatically restart check box, and click OK the necessary number of times.

5.Restart your computer for the settings to take effect.

Now when you go to shut down and a fatal error occurs, you'll at least see it and it won't cause an automatic reboot. then we can sort out what's causing the problem.

As owbist was suggesting it may be heat see the link not sure what model you have..

Would like to see the Blue screen error also...

Here is a link that might be useful: Toshiba heat problem

Yes its a laptop, but it sits on a cooling tray that the fans keep it cool.
Thanks for the info, I will probably let my husband do it, so I don't mess it up.

If you happen to have pets, the fans/processor area will accumulate obscene amounts of hair compacted against the heatsink. The only real way to get rid of it all is to disassemble to the point of removing the keyboard and cleaning in those areas.
A local authorized Toshiba repair shop charged about $50 to do this. By the way how old and what model is your laptop