Hi Durk,
How about PS/2 keyboard and mouse? I suspect the problem is related to the keyboard/mouse driver, or there is conflict between the driver and Windows SteadyState. We can disable SteadyState related services from Recovery Console to check this issue:
1. Insert the Windows XP CD-ROM into the CD-ROM drive, and then restart the computer to boot from CD.
2. When the "Welcome to Setup" screen appears, press R to start the Recovery Console.
3. Type in the following command and press Enter after each line:
Disable "Windows SteadyState service"
Disable vcfsvc
4. Restart the computer to Normal Mode or Safe Mode to test.
How to install and use the Recovery Console in Windows XP
http://support.microsoft.com/kb/307654
Best Regards,