Keyboard Lock-up at Windows log-in or installation!

Day after day, I encounter unusual problem on the field, such as this one:

The OS: Windows XP

Problem Found: Keyboard lock-up when you you reach at log-in window(Cant type anything), but mouse is functional

Troubleshooting Done: Check keyboard on DOS mode, its working fine. Try to replace a USB keyboard, problem still exist. Try to fresh install OS without reformatting,still the same problem.

Solution: Reformat the hard drive, problem disappear,the problem was cause by a problematic OS.