there are 2 causes to this. Obviously the first is ntoskrnl is corrupt or missing, to fix this do the following (I love google, instant answers to stuff I don't remember

):
1. Insert the Microsoft Windows XP CD. Note: If you have a recovery CD or a restore CD and not a Microsoft Windows XP CD it is likely the below steps will not resolve your issue.
2. Boot from the CD
3. In the Microsoft Windows XP setup menu press the R key to enter the recovery console.
4. Select the operating system you wish to fix, and then enter the administrator password.
5. Type expand d:\i386\ntoskrnl.ex_ c:\windows\system32
6. You will then be prompted if you wish to overwrite the file type Y and press enter to overwrite the file.
7. Type exit to reboot the computer.
The other posibility is your boot.ini is broken.
1. Insert the Microsoft Windows XP CD into the computer. Note: If you have a system recovery CD or restore CD these steps will likely not work for your computer.
2. Boot from the CD
2. Once in the Microsoft Setup menu press R to open the recovery console.
3. Select the operating system you wish to use; if you only have Windows XP on the computer you will only have one prompt.
4. Once prompted for the password enter the Admin password and press enter.
5. Once at the command prompt type bootcfg /rebuild to start the rebuild process.
this will take a while.
6. Type exit to reboot.