Keyboard disabled

I am trying to format a hard drive and reinstall windows xp, but whenever I get to the part “press any key to boot from cd-rom” my keyboard is disabled! It works at all other times, including when I first boot up (I can go into the BIOS using the “del” key) and when I am in windows! Can someone please tell me why it is disabling itself, or how I can boot up from my windows xp cd without having to press a key? The keyboard is a usb microsoft office keyboard (the ps2 ports aren’t working so Ican’t use a ps2 keyboard).