I wouldn't use any shit PC to emulate any old computer sound. I wouldn't use any other type of synth to emulate the computer sound either. The results won't be near as good. But if it fills your bill... Try some squarewave synthesis keyboards from the 80s.
What would be the problem in you sampling the real hardware and then sequencing in the PC?
Anyway fruityloops is not bad at all, at least I used it a lot until I got my Mac. A nice program that also lets you sequence MIDI in case you have any other sorts of external, MIDI-controllable devices. You can use the 3xosc instrument to try and pull some oldcomputerness shit. The TS404 can pull some vintagecomputer-like sounds too.
Modplug tracker is pure shit and it will only let you make mods. It won't let you synth anything so you are at ground zero with the sound making. If you are still in for a tracker use ReNoise or Mad Tracker. Or wait for Protracker II
Never tried Buzz but seems to be a poohman's Reaktor, which is not half bad. Should suffice on the synthesis side.
I don't know what expectations you have with yoru music making but I'd say Fruityloops is a pretty nice tool to start up and milk for quite some time (I still use it from time to time)
OOps forgot to say what I use. Fruityloops sometimes, Reason a bit, and on the Amiga, AHX and MusicLine. CyberTracker for the C64 (you could use GoatTracker on the PC), and LSDJ on the Gameboy.