If you get a message that this file is missing, it's not really just that file. Usually, you are trying to boot a corrupted install or even boot from a partition with no install.
It is a very good idea to back up your whole disk before proceeding, if you have a bootable backup utility. If not, download one first. EASEUS gives away a free version for cloning, and Clonezilla seems to be widely regarded.
Here's Microsoft's take on it:
http://support.microsoft.com/kb/124550
Search our own forums and read the many threads on this topic, to save the members the trouble of typing it all over again:
http://www.tomshardware.com/search.php?s=+system32+ntoskrnl+exe
This is a mildly interesting take on it, including a keyboard fault(!):
http://www.computerhope.com/issues/ch000646.htm
Let us know if any of these help you, or what the circumstances are that make this not respond to the usual treatments.