Cannot logon after SP4 Upgrade

Hi All,

We have a Latitude D600 running Windows 2000 Server. I tried to install SP4, and it seemed to go OK, but after rebooting, hitting Ctr Alt Del and putting in the password it sits at a blue screen and will not go any further. Further Ctr Alt Del's have no effect, all I can do is power off.

I can log on in safe mode, and can uninstall the SP.