You haven’t given much info for anyone to give a helpfull answer
Can you post again and give as much info as you can with details of any error messages and details of what has changed between the time the machine was working ok and not
This should also include details of any software you have either installed or un-installed
This page may be a helpfull starting point
If not, use the search function to find a knowledge base artical that describes your problem
http://support.microsoft.com/kb/831431/en-us
Another way is to google for Windows shut down problems
J