To fully help you, don't just tell us "there's a blue screen of death." Write down what you can and try to get the error message in there.
A shot in the dark because we don't have an error message to go off of: It might be a RAM problem, I had it before. Try taking out one of your RAM sticks and see if the problem occurs, then try the other one and see what happens.
It's also scaring me that you said you built this computer yourself, but don't know what speed your processor is at...