TTCM - have you ever pulled a computer apart? They're really not that complex inside. With a phillips screwdriver and 30 minutes you can replace just about anything inside. If you've got even a shred of mechanical ability, you can figure it out just by looking.
Usually 3 to 5 screws hold the cover on the PC. 3 hold the power supply on the case. 2 connectors to the motherboard (pull the clip away from the connector, slide the connector out of the socket). 1 connector to each drive (floppy, CD, tape, etc). Put in the new power supply, making all the connections you took apart.
Make sure you've unplugged the thing before you open the case. Just to be safe, to avoid static discharge frying any of the components, keep some part of you body in contact with a metal part of the PC case the whole time.
All this is assuming you're working on some Intel type machine - I've never worked on anything Apple, so I don't know if the instructions would be the same.