First, try system restore if you have been periodically saving good installs.
Usually, you an boot to your windows CD, and run Windows XP in repair mode... but this could be malware as well.
The computer believed that this file is corrupt: C:\WINNT\System32\Ntoskrnl.exe
This is often caused by a bad hard drive that is beginning to fail. On some computers it is caused by a defective keyboard. But it can come from any set of corrupted files. Some times it is the boot.ini file
But the problem can be more complicated if your computer came with a Recovery or Restore disc instead of a Windows XP install disc. If that is the case, you are better off reformatting and reinstalling Windows after you save the data... for which you will require a separate disc.
Using a regular Windows XP dis, you boot to the disc, then press the R for repair to enter the repair or recovery console.
Select the operating system, then enter your admin password.
Type: Expand d:\i386\ntoskrnl.ex_ C:\Windows\System32.
You should get a prompt asking you if you want to Overwrite the File.
You should enter Y for yes, the press <ENTER> to Overwrite.
If that works, you are probably home free.
To know for sure, shut down the computer, then run a cold boot with the <Start> button.
If it starts, correctly, then you are done. And lucky.