Welcome to BC!
Go to:Start > right click on My Computer > Properties > Advanced tab > Startup and Recovery Settings
Uncheck Automatically Restart
and then OK
Now when the computer has a problem it will give you a blue screen and you will have to reboot manually.
The 'Blue Screen of Death' (or BSOD) is refered to by Microsoft as a Stop Message. They can occur for many reasons. In order to diagnose it's cause we need as much info as you can give about the stop message. Please write it down and post it back here the next time it happens. In particular, it helps to have the somewhat cryptic series of numbers about half way down the blue screen that have this sort of format:
0x0000008E (Followed by four other sets of numbers in parenthesis.)