I have been having this problem for a while now. I have an Alienware M7700, running Windows XP Media Center 2002 SP3 X86. The keyboard I am mainly trying to use is a SteelSeries (formerly Ideazon) Merc Stealth keyboard. I have used the keyboard in other computers and it installs and works in all of them. I tried other USB keyboards that I know for sure work, and they do the same. All the keyboards work in the Bios, and there is no option to enable USB legacy support in Bios. I tried replacing the keyboard.inf file from a computer the keyboard is known to work in, and no success. PS/2 keyboards work fine. Thank you for all answers.

That sounds like a driver issue to me...Driver files are the items that actually Identify a product. If something is loading wrong then in most cases it is the driver that has been corrupted...Try uninstalling the old drivers for the thing and downloading reinstalling the files.

Downloads - SteelSeries