You need to borrow a WinXP CD from a friend, then :-

In the following commands, C: represents the hard disk and D: the CD/DVD drive, change them as necessary.

Boot the XP CD and select the first repair option and then copy ntoskrnl.exe from the CD to your hard disk as follows :-

At the prompt, type copy D:\i386\ntoskrnl.exe C:\Windows\system32\ntoskrnl.exe (note the 2 spaces or copy/paste the statement) and press Enter.

If you can only find ntoskrnl.ex_ then the file needs to be expanded by typing
expand D:\i386\ntoskrnl.ex_ C:\Windows\system32\ntoskrnl.exe and press Enter.