'unrecogniseable usb device'

i have 2 nano style mp3/4 players
1 was transferring music from my pc, then i plugged second one in and windows wouldnt recognise it, now both players cant be recognised by windows, it says 'unrecogniseable usb device'.
any one know how to uninstall the drivers that windows installs when you plug the player in for the first time? so they will reinstall when i plug my player in.

thanks

unjust