Windows won't start

A few weeks back, my uncle gave me his old laptop. Its a Dell Latitude D505.
I've had no problems with it until yesterday, I opened it, turned it on, and after trying to boot, I got a message saying "Windows could not start because the following file is missing or corrupt :
<Windows root>/system32/ntoskrnl.exe.
Please re-install a copy of the above file."

and I have no idea what to do! So I hit the space bar, and it just tried to boot again and then did the exact same thing. I ran a diagnostics test, I tried to boot it using my Dell desktop's CD, but that didn't work, and I don't have the CD for this laptop because it is very old and used to belong to my uncle. The laptop will not boot.
I just want it to be fixed, I need it for school! :confused: please help!
 
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