Using an old Windows keyboard on a Macbook
I have heard that all USB/PS2 adapters are not created equal. Some work some don't. But you would probably cost yourself more time and effort getting a working adapter than just buying a cheap PC USB keyboard.

I use PC USB keyboards on Macs all the time and they work a treat. I can get a used (Dell) USB keyboard locally for about twelve bucks. As cheap as they are you could spend that on two adapters easily, not to mention time spent futzing with it...