how to set up a dedicated cod4 server on an old computer

Yeah you should be able to run it fine on that hardware... the server doesn't take that much... infact it will only use about about 400 meg max per server... the CPU power will be the downfall here but you should be fine... also your going to want to have a good connection if you plan on having a bunch of people on it.

As for getting the server up and running... check the charlieoscardelta.com forums or the infinity ward... it has been discussed many many times over...

Go to speedtest.net and check your UPLOAD. Not sure what it's supposed to be per player for CoD4, but for BF2 it's about 80kb/s (which should give you some leeway, seeing as that game has more objects in game to track).

So if you want a 10 player server you'll need 800kb(it)/s, constant while the game's in progress. You can get some nasty lag spikes on home connections though. Give it a bit of extra headroom, incase everyone else on your contention block decides to hit the same youtube vid at once.