Windows xp usb devices not working

I have a desktop that runs XP but it used the older PS2 type connections for the mouse and keyboard. those connections recently stopped working, so i tried to use some usb devices. windows recognized that there were devises plugged in, but it said that i have to have the original XP disk, and I no longer have access to that disk, so i tried to restart. bad idea. now the computer will not get past the Welcome screen for i have no mouse and/or keyboard to get past it. What do/can I do?