You can get emulators to play ps2 and ps1 games on pc, using keyboard or dualshock connected via usb bridge. I'm unaware of any means of playing psp games on it, and ps3, at least for now, is a definate no-no. for that, you'd need a blu-ray drive fitted into you pc, and a ps3 emulator, which is as yet not available.
There are various emus available, but i havnt (in the hour i tried to) found one that comes fully equipped to work straight away. i hav a ps3 anyway, and as such are'nt that intrested in finding one. try googling "ps emulator" or "PSX emulator" and try a few results.
PSN ID: DaNrOx48
Games: Army Of Two, Assassain's Creed, Blazing Angels: Squadrons of WWII, Burnout: Paradise, Call of Duty 4: Modern Warfare: Guitar Hero III: Legends Of Rock, Harry Potter and the Order of the Phoenix, MotorStorm, Resistance: Fall of Man, Skate.
To Buy List: Haze, GTAIV, LittleBig Planet, Motorstorm 2, Resistance 2, MGS4