OK..let's try to narrow it down?

If your keyboard is USB and your mouse PS/2...which is how I have understood your posts?

1. Clean the USB connector with isopropyl alcohol (screensparay/CD cleaning fluid) and try it.
2. Boot the box WITHOUT the USB keyboard, then plug it in afterwards and see if it is recognised. Try both USB ports.
3. Borrow a PS/2 keyboard and try that....see if you get a 303 error.........I would be very surprised if you do not have a PS/2 connector............I have never seen a MoBo that did not have one.

If 1. works, the problem is solved
If 2. works, then go into the BIOS/CMOS setup, on boot and disable "USB legacy support for DOS"
If 3. works, then your USB ports/keyboard are defective (this may be intermittent?)......if you get a 303 error with a PS/2 keyboard, then we have to consider the possibility of a boot virus?

Have you tried to boot from a floppy/CD, or in safe mode?

Please let me know what happens and we can take it from there

Cheers