You could plug the HTPC into an x10 appliance switch. Then run an automation program like Homeseer,
http://www.homeseer.com/ which has a web interface. Log on to the web interface using the PPC browser, click on the Homeseer interface link to turn on the HTPC remotely, Homeseer sends the command through the powerline to the x10 switch, and voila, remote switching from anywhere you can get to the the internet. If you just want to do it locally, there are various x10 devices which will receive IR signals, and convert that to an x10 powerline signal to be sent through the powerline as above. Just use a remote control program on the PPC that sends out IR.
Homeseer could be set up to do this whole scenario based on just about any event: IR received, email received, specific times of the day, motion sensor tripped, you name it, this program is very versatile. Maybe more than you need for your needs.
good luck
Brent