We need more info on this system:
What is Make/Model info?
What upgrades have been performed to it if any?
What leading up to this may have caused this problem to occur ( such as malware or virus was removed a short while back, or the computer was opened and something was done while open which may have introduced a problem )?
What OS version are you running on this computer?
Does it freeze randomly or only when a specific action is taken?
The problem is that a number of problems can cause a computer to freeze up and without specifics its a guessing game.
