Error \system32\ntoskrnl.exe. ?

I got this laptop, when i turned it on it went to a blue screen. after 3-5 mins it went to a screen that said:
The following files are missing or Corrupt:
\system32\ntoskrnl.exe.
please Reinstall the file above.
this is really Frustrating please help...
also this laptop isnt mine :'(
Also-- My father took a look at it. he ran a Diagnostics test on it. he got to it when he pressed F12. took about an hour for the full scan to get to 31%, then when it got to about 28% of the test below it failed that whole test
IDE Disk S/N=NNI ST441GJEJ- Read Test
it kept failing about every one after 28% but then after that test it passed every other one,
guess it was just that test that was messed up.
**Do you think me playing with a magnet on the dash board messed it up** i know but i was bored
PLEASE HELP i dont really want to spend money to buy my brother a new laptop, he needs it for college next school year :/