The 3rd generation were the ones that Apple shipped expressly for Windows with iTunes for windows. However, they had gen1 and 2 working with Windows with MusicMatch - see
Installing MUSICMATCH Jukebox Software
The apple retail stores sold Belkin Firewire PCI cards expressly so that iPod users could work with Windows.
Gen 1 and 2 will both work with OS 9.
Gen 1 = 5, 10GB, physical scroll wheel.
Gen 2 = 10, 20GB, touch wheel with physical buttons surrounding it.
Gen 1 and 2 suffer from a few problems. Gen 1 will have the wheel get too loose. I found that removing the wheel (place masking tape over wheel and then pull tape to lift wheel off without marking it) and packing the bearing with marine teflon grease made it spin, but with the resistance similar to when it was new.
Gen 1 and 2 both suffer from strain on the firewire port from the cable, and this can break the solder joints for the firewire connector off the logic board. I have resoldered a few of these.
Gen 1 and 2 both suffer from the ring around the headphone jack for the remote control breaking. I have superglued or replaced the jack.