Win7 Problems - Computer Shuts Off

First off, I apologize if this has been asked. I tried searching, but didn't find anything that I was looking for.

I installed Windows 7 64bit onto my computer yesterday which was running Vista 64bit. Everything seemed to have worked okay, until about 10 minutes later, the computer shuts off as if it loses power. When I went to turn it back on, the fan is extremely loud and it sounds like one long beep and two fast after it.

There is also no signal on the monitor and nothing shows up. I have made sure many times the wires are plugged in. I tried flipping the power switch on the back of the computer as well, and that still failed to work.

If it helps, I own a custom built computer. But I know nothing about computers, so please try to explain as though you were talking to a five year old.