The IR transmitter and receiver are the most fun part as there are so many transceiver ICs to choose from these days. Once you get your ON/OFF output from the receiver, you might as well just use a solid state relay to do the switching. There are many with 5V input that switch 120V mains.
On second thought, aren't most computers using soft ON/OFF switching these days, in which case just switching the line on/off won't help. I guess you need a simple relay in parallel with the pushbutton on the front of your PC. Relays may be old fashioned, but at least they are isolated so easy to patch in to an existing circuit.