Please help - a Windows problem

A friend of mine's PC (XP service pack 3 *shudder*) has a problem - when he connects a USB stick the PC hardware Blue Screen of Deaths, a known problem with Windows XP Service Pack 3 (yes, he used Auto-Update, which I detest).

That isn't the problem I'm posting about (though any thoughts on that problem would be appreciated).

See, one of the solutions I found on Google was to delete all of the USB drivers from Device Manager, then reset the PC, and all of the drivers would be reloaded by XP and the problem would (alledgedly) be cured. So I saved a Restore point, and deleted all USB drivers, and reset the PC. And Windows XP went to the log on screen, and goes no further. See, the mouse is a USB too, and so I can't log onto XP (it has five user profiles) without a mouse, but I can't get control of the mouse until XP is logged in (when it will then detect and load missing drivers)...

I can't log on via the keyboard, as nothing I've tried works. Can you log on via the keyboard? You'd think so, but nothing I press does anything. Surely even Microsoft would have made the log on screen controllable via the keyboard as well as the mouse? And the keyboard doesn't have any lights (Caps lock, Num lock, etc) so I can't even be sure that the keyboard is working at the log on scree - the keyboard works in the BIOS and when I boot up with the Windows install CD, but perhaps for some reason Windows doesn't read it at the login screen? The reason I suspect this is that after a time, the screen saver kicks in on the logon screen, and no matter what I do with the keyboard or mouse, the screen saver won't go away.

And I can't get into Safe mode, as that too greets me with the log in screen. And I can't try other keyboards or mice, as the PC is USB only, whereas all my keyboards and mice are PS2.

Thanks for any answers.