Welcome to the forum
First, please read here.
The more details we have the better we can help you.
What is the brand and model (and model number) of your laptop? Although 12.04 might be too new for it, I kind of doubt that's the reason why it's locking up. Did it run ok when you had Windows XP installed?
In terms of some things to look for, please see my post (#2) here.
As Emerson pointed out, overheating is a prime suspect. Everything in my list will not apply to you or might be too difficult. Please report back with what you can.
Also, just as a suggestion, you might want to change your username using the instructions at the top of the page here,
lest your email inbox ends up with a boatload of SPAM in the very near future.