more than likely its somethig starting up with windows that is maybe causing the problem.
click start/run type msconfig click ok.
click the start up tab and make a note of items listed here.then untick everything listed here and restart the computer.
does it start up ok?
if so go back to msconfig and add one tick back at a time and restart each time untill you get a restart/hang etc.
you may find the problem.