software sure?
if your just going to use the computer as like a mp3 player or server, you can just install pcAnywhere 9.2 (works on win9x, winNT, and win2k) Just hook ur monitor up install pcAnywhere on the computer you won't be using much. Then stick your monitor back on ur other computer, install pcAnywhere on that, connect to your other PC bam! It works VERY well, its just like your on that computer seriously.  As long as you won't be playing any games or anything on that computer...  Its pretty low bandwidth too.. Ive used it over the net, i can connect to my computer and run it fine over 56k connect as long as your in low-res...
I have it installed on all my computers, its great, has some good security features too.  just make sure the computer your using to remote other computers with is at least same or better resolution. so u don't have to scroll around and stuff.