Can you charge the phone on the computer? If not, I hope you have enough of a charge.
First, get into download mode (vol down + power). If you can do that, and I'm presuming you see going to load a ROM, copy over a custom recovery (revovery.img like a version of Clock Work Recovery).
Then, assuming you have the ROM on your SDcard, pull the battery.
Then boot into recovery (both volume up and down + power) and flash your ROM.