Opening your computer and replacing the PSU is easy. Make sure you are grounded first, then simply open up the computer case and take out the PSU with a screwdriver. It's just a box that slides in and out of the case with a few easy connectors. You can buy a new one at Best Buy or any other store that sells components. Check the power wattage on the one you had and get an identical one so you have enough power (and don't blow out the motherboard). Hooking it up isn't too big of a deal if you follow the instructions.
I don't know if it fried the motherboard or not, but buying a new power supply and testing it out is a cheap way to find out. I suspect it didn't hurt the motherboard. When motherboards die they make a horrible sequence of long beeps which, as anyone who has heard it will attest, is unmistakably a death rattle. I heard it once and it was so obviously finished that I couldn't stop myself from laughing at whoever programmed my PC to sound like R2-D2 falling into a giant trash compactor. There's no doubt when the motherboard is dead.
While you have the case open you should also check that all the fans are clean and operating properly. Also check the area where you've put your case. Make sure there is enough ventilation for the unit. The failure of the PSU may be a result of old age, but it may also be a victim of environmental factors.
