I have a multy boot system and all of the sudden I'm not able to boot from one of the Operating systems. I got the following error message:

Windows could not start because the following file is missing or corrupt:

<Windows root>\system32\ntoskrnl.exe.
Please reinstall a copy of the above file.

Any idea on how can I solve this? Any help will be very appreciated.

Well there are 2 versions of that file. And they both carry the same file name. However, if you right click on it and see its properties you can see
that one of them will have the internal name of ntompkrnl.exe or something. Anyway, 1 version for single core system and 1 for multi core/processor system. That might be what has happened that this file has gotten overwritten with the wrong version somehow.
Or you have used a tool to create custom boot screen and it didnt do it properly.
I've also seen xp install these files on a totally different partition then systemroot, because somehow in the installation process windows decided that was the primary active boot partition or what not. Makes no sense to me either.

IN ANY CASE, i'd just get hold of a boot cd and copy files to proper location, and if all else fails, i'd boot with a windows cd and do the recovery thing.

If you have many OS like me, you should get a decent boot manager program, also it is best to install oldest OS first, then newer and newest last.