Offhand? I've never tried carrying a desktop around in a backpack. I didn't have a fork truck around for the old tape driven IBM mainframes in the late 60s either. sorry!
You will have to start researching 12v battery backups for pcs or something on that idea to see what you can come up with. Even those that go to lan parties at least have ac power avaialble or simply go for a laptop to make life far easier. You still haven't explained what you are using for a case for this.
well my laptop. duel core turion 64 at 1.8 w/ a 17 inch monitor for 3 hr on a 14.4 V 63 Wh battery. so i mean you could get a good 12 cell battery and may run it for 3 hr. maybe.