As far as Nintendo goes, there's Nestopia and Snes9x which are both free. There's also sixtyforce for N64 games, which is shareware. I've tried all three, Nestopia and Snes9x extensively and they worked great though a USB joypad would help make the experience better (especially for the n64 games).
As for Sega, no idea!
You can download all three Nintendo systems games (or ROMS) and Sega ones too free on the old computer