Errr, all computers running Windows are multi-tasking. We're not talking about the iPad here ya know.
To really answer your question however, your typical mid- to high-end CPU does a great job of multi-tasking on its own. Technically a server would do a better job of managing applications—if they're written properly for the server's architecture.
I Have duel monitors and nvidia 8600 GT that they hook to WATCHING A MOVIE ON ONE AND EMAILING AND playing a flash game they just seem to confuse the computer no lock ups just waiting time more than I can stand, with all three going in win 7 pro