I know this thread is pretty old, but my dad's Dell XP machine had the same issue. Keyboard (USB) worked in Bios, but not responsive at all at the login screen. Mouse was powered (USB), but didn't work at login screen either. No PS/2 ports on this desktop.
So I was reading this thread about the i8042prt.sys fix and had to use expand.exe on i8042prt.sy_ file from a XP CD I had, then save to flash drive ... then went back to the other room with this Dell, and the keyboard and mouse were working.
I didn't have it on my network since I knew it was full of viruses and malware, so it didn't download any keyboard/mouse drivers while sitting there.
I guess what I am trying to get at, is if this happens to anyone that finds this thread, to let it sit at the login screen for 15 minutes and see if it helps.