Right click the "My Computer" icon and select properties.
On the "Advanced" tab choose the start up and recovery settings and uncheck the "Automatically Restart" box.
Next time it encounters a critical error it won't reboot, it will flash to a blue screen that will give you a stop error at the top. Write down the entire error code, go over to the MS Knowledge Base and plug the error code into their search engine. That will tell you why it's doing it.
If that doesn't work, go make yourself some blue eggs, grab a beer and relax.
