we did that useing Unreal Tournament,some how just playing it broke there net work.
did you go to my school. We had the UT demo installed on our network (by a lab technician). When the teachers had had enough they deleted the short cut. So someone copied another short cut an emailed it to everyone. UT demo only has about 5 maps but everythings fully functional for lan play and the specs are low.
When i first started secondary school (at a different school to the one above) there was an old pc room running win 95 machines. There were disks circulating for doom, lesure suit larry, wolfenstein, scorched earth and some other games.
id recommend scorched earth or worms. worms can do hot seat as well as lan i think, (also hedgewars a new sort of remake).
also hardwar might be worth a look, you can download it free and its a spacesim. good multiplayer, only you might struggle for hard drive space.
i second defcon
also gang garrison - like soldat but teamfrotress demake
possibly the original avp, u can pick up gold edition pretty cheap but ud need to find a crack to get it running on multiple machines.
hard drove space and the baility to install will probably be more limiting that graphics/cpu. In the uk at any rate most even old pc rooms with be post 2000. which will mean 800-1000mhz cpu and 32bit graphics. If you and your colleagues can afford 500-1gbpen drives cost about £8 $13 each (maybe lessO you can install the games on the pen drives at home (aslong as the drive is formatted to fat32) then just plug em in alsong as the games are either cracked or have no protection you can run em off the exe.
also red alert, thats now freeware
and shogun total war