<windows root> System32\NTOSKRNL.EXE missing

Hello,




When i boot up window xp i get a error message that it says <windows root>
System32\NTOSKRNL.EXE is missing or corrupt.i tried a lot of method like safe mode,safe mode with networking.eventually i solved nothing.although i follow the method of accessing windows root e.g. Type: expand:\i386\ntoskrnl.ex_ c:\windows\system32.but i still can't solve the problem.what should i do? I refer many websites and videos from youtube yet can't solve it.Please help me and I don't want to lose all my important files.