The file u mentioned "<Windows\System32\ntoskrnl.exe" is the file, which required during the boot time.
And somehow it has been deleted?
What u can do is:
1.) Either repair ur windows using Setup CD
2.) Goto "Windows\System32\Dllcache" folder and copy the ntoskrnl.exe file to "Windows\System32" folder
3.) Extract the ntoskrnl.exe file from the Setup of windows XP using Extracter utility like WinRAR
4.) d/l ne boot screen from sites that provide these and copy the file in "windows\system32" folder

( U'll get a new boot screen too )