ya hard to fix a problem you can't see...I went BSOD and figured this out too
so I do this to every system I install now
assuming you have access to the system...
1.Go to Start -> Control Panel -> System
2.Go to Advanced
3.Under the Startup and Recovery section, click Settings...
4.Under System Failure un-check "Automatically restart"
But you probably can't do that yet will have to check the error log to see
the error code, you may have an idea already so you should use someone elses
computer to make a Previously-Installed Environment (PE) which emulates the
Windows OS environment from a CD or DVD. Some also have the ability to add
antivirus , diagnostics tools etc to the cd.
Barts PE is the most common. (I actually just made one today from the same
problem)
http://www.nu2.nu/pebuilder/download/