Here are a few ways to do it:
If you have an IR keyboard and can Power your PC from the keyboard (the BIOS of many computers lets you do this), you could teach any learning remote the IR code for the right key presses.
If you have a UIRT2 (this wont work with the newer USB-UIRT), you can use any IR remote and teach the UIRT2 the power on code of your choice (see
www.girder.nl for more information on how to configure everything)
If you can wake up your computer via a network (WOL), and have NetRemote or any other networked remote, you can send a wake-up packet to your HTPC
Finally, the last resort solution, IMO, is to use X10 appliance modules, and an X10 IR receiver such as the IR 543
