Please help - keyboard and mouse don't work at windows login

I am running windows xp sp 3. Last night i was working on it fine, then shutdown my computer. when i booted it back up later on the keyboard and mouse wouldn't work on the login screen. When booting i can get into the bios or send it to boot from the windows xp cd, but once it gets to safe mode, regular windows, or even the windows cd boot screen (where you can select to repair windows, format, etc.) the keyboard and mouse stop working again. anyone have any ideas?