Actualy theres 2 solutions:
Hybrid hardware/software soution there is the ApplicaDS (Windows) , Axel AX3000 (Windows and Linux), Wyse Thin teminals (Windows and linux), Userful (Linux), and multiseatcomputer.be (Ubuntu and Debian), some of that solutions connects thru free USB ports, other thru LAN card, other thru free internal PCI slots, and other are simply a complete mainframe/terminal solutions.
100% software solution (your PC must have enought free USB ports for mice and keyboard and a PCI or AGP dual monitor ready videocard), can use BeTwin under WXP or Vista (need aditional USB soundcard for each aditional user, if ýour PC doesn't already have a dual monitor ready videocard can use a dual or quad monitor PCI or a AGP videocard, I don't know if works with PCI-E videocards nor if is W7 compatible), Aster XP 4.5T (free for 2 users under WXP), sotfXpand (Windows) or simply can use the FREE multiseat function already included under Ubuntu and Devian...
The problem with most Windows solutions (hybrid hardware/software and software only) is that they can run only aplications like internet, office suits, media players and can´t run programs that use intensively OpenGL like games, all because the share of the videocard.