What file is effected if someone hacks into system? Is it NT OS kernel?

I wonder if I have been hacked.  I met someone a few years ago from a local IT company that told me that if someone is determined enough, they will get into any selected computer.

 

Well, a few weeks ago, I had my "ntoskrnl.exe" file become corrupted on my Win XP SP2 computer.  Note: I do have a Linksys wireless router (WRTG54S) and one of my Windows Vistas networked computers is hooked up via the wireless with a wireless card.  I did encrypt it sometime this past September.

 

Would the "ntoskrnl.exe" file have been the one that someone would have used to either take control of my computer or just watch me? 

 

Note: somehow the ntoskrnl.exe file magically fixed itself without any of the normal troubleshooting required.