How do I boot one comp from another?

Answer 1

The short of it is, you can't. There's nothing in a normal computer that allows any kind of network connection to run while the computer is off. And that network connection would be required to control any resource remotely. That's not to say it isn't possible but, you will need some additional hardware in order to make it work such as a power supply that can be controlled remotely over a network.

Answer 2

If you can use RDP to access the computer to be booted [would have to be running XP Pro or Win Server 2003] open a command prompt and use the command:
shutdown -r

or to just shut it off use:
shutdown -s