System BIOSes have been containing legacy keyboard emulation for USB keyboards for some three to four years now. Operating systems that use BIOS services for keyboard won't notice. This includes DOS, Windows 9x/ME until booted, BIOS setup itself. The only kind of OS that won't be able to use the USB keyboard are those that access legacy keyboard controller hardware directly AND don't know USB. This is of little relevance on the desktop though, since it only affects oooold Linux distributions and equally old Windows NT 4.0.
Note that system BIOS only does this kind of emulation on the chipset integrated USB controller.
regards, Peter