LEXCERM

Hi all, A problem has arisen which I've never experienced before. When logging in, the blue welcome screen appears, the hourglass flickers, and the PC freezes. It doesn't even take me to the login screen. I've noticed that when hitting ALT TAB, I have two options: WINDOWS LOGON & LOG ONTO WINDOWS, but nothing happens. Tried booting in SAFE MODE, LAST CONFIGURED SETTINGS etc but to no avail. SAFE MODE brings up a black screen with a heading SERVICE PACK 2, but it then goes to the welcome screen again and it freezes. When hitting F1 when re-booting, a message regarding the KEYBOARD appears vry briefly. I've swapped for another good keyboard, but to no avail. Any ideas? Thanks in advance.