First, don't tap into the radio power harness. Those wires are very small (18GA or less) and not designed to deliver the kind of voltage needed for your computer power supply. You need to run a dedicated 12V line from the battery to your computer, using at least an 8GA wire (I'd go 4GA). You will also need a fuse installed within 18" of the battery. You can buy an amplifier wiring kit (buy a good one - you don't want a crappy power line) that will have everything you need, or you can visit your local car audio store and they'll give you everything you need.
In the short term, you can move your computer to your engine compartment and connect to the 12V terminal and the common ground on the chassis to test the power issue. You should also review the manual and make sure you connected everything correctly and set the jumpers correctly.
If you don't know how to run the wiring, this is a good starting place: http://www.caraudiohelp.com/how_to_i..._a_car_amp.htm