google wake-on-lan and magic packets. if you go to device manager and look up your network adapter and go to the advanced tab there should be a wake on lan section or wake up capabilities. just set it to accept magic packets and just use any free app on the net to send a magic packet it to the pcs ip address when its turned off, and it should turn on.
this has only worked for me on the internal network so you can't for example do this from yer iphone from halfway across the world, but if you login to your home server should be fine.