There was a time when if you bought a USB mouse, in the box was also an adapter to allow it to be used with the older PS/2 interface. And if you were to go back a few more years into the past ...

The open source HIDman adapter debuts, for interfacing modern USB peripherals with old PCs by emulating classic PS/2 ...

[danjovic] has you covered with the open-source EXPS/2 keyboard adapter. It will allow you to connect either a PS/2 keyboard or a USB keyboard in fallback mode to the Expert using an ATMega328P ...