If you want to be able to log into your PC as different people, then just use normal user management. Only one user can be logged in at once and each one will have their own desktop etc which may or may not be visible to other users, depending on your security settings.
If you want to connect to a desktop from other machines, then (presuming a Windows environment), Terminal Server is your app of choice - there are several hacks around to enable multi-user TS on XP / Vista / 7, so you don't need Windows server to achieve. This enables you to have several dumb clients connecting to one machine, sharing it's programs and resources.
If you want 2 screens, 2 keyboards and 2 mice on the same machine and have them work independently? No idea, sorry