I'm trying to help my brother figure out what is wrong with his computer. It freezes up when he tries to turn it on. Sometimes it will do nothing, sometimes it makes it to the windows vista load screen and freezes there, a couple of times it got to the main desktop before freezing. We know its not the HDD or memory, but that's it. Does anyone have an idea what might be broke?

Try booting from a CD (IE Ubuntu or some other linux live cd). You can also do a full memory test from some of those (Ubuntu I know has it).

This can help rule out the HDD entirely as being the cause...

Question:

How do you know it's not the hdd or memory? Sounds like it could very well be bad memory. Could also maybe be an overheating processor? Make sure the heat sink didn't come loose. Based on when it's freezing up, I'd say it's almost certainly a hardware issue.

the hard drive and memory have been replaced and checke dout, so we know they work. I thought it migh be a problem with the power supply

(i think) power supplies symptoms would make it restart, not freeze. Not sure there though.