Many newer computers have a feature called "Wake on LAN" that allows another device on the network to turn that computer on remotely. It's intended to allow network administrators to do upgrades and backups at night.

The IRA-3 is an inexpensive device that lets you do this via an infrared remote. I haven't used this myself, but have considered it.

See this link for more information:

http://www.home-electro.com/

Cordially,

Eric