I have a 'USB to PS2' dongle thingy with a USB mouse (into a PS2 adapter!) and keyboard. The trick is not to buy a keyboard PS2 adapter... buy one that does BOTH keyboard and mouse. These work better than the single sockets for some reason.
Most of the single-socket adaptors that come packaged with a keyboard or mouse just make the physical connector fit. The chipset in those keyboards is designed to talk over whichever protocol it finds on the wire.
As a pre-USB-era keyboard won't have these kind of electronics inside, the adapter that comes with most modern keyboards won't work. The double adapters that you can buy separately, on the other hand, actually have electronics inside to translate the PS2 signals from the keyboard and mouse into USB.
