Windows 2000 Adv Server : ntoskrnl.exe

My win2k adv server crashed and I get the error message: Windows 2000 could not start because the following file is missing or corrupt :
\system32\ntoskrnl.exe
Please re-install a copy of the above file

I tried to fix this by using boot up disks with repair option. I copied ntoskrnl.exe to \system32\ fron w2k cd using expand but when I reboot, I still get the same error

Can anyone plese tell me why this happens and how can I fix this ?.

Thanks