wont reboot now ntoskrnl.exe

Further information:

The virus or worm had now corrupted file ntoskrnl.exe, as my PC (windows 2000) would not reboot and indicates the file \winnt\system32\ntoskrnl.exe is either missing or corrupt.

Tried fresh installation of original windows 2000 and applied the various patches, however, this worm/virus had come back again - reformat my disk would be my last resort.

Any tips?