What you could have done to be safe was to use multiple boot. Keep the orignal ntoskrnl.exe in sytem32. You should save your new ntokrnl.exe as say ntosboot.exe and save it in system32 folder.Now in boot.ini copy and paste the line already present so as to make 2 lines.
for e.g this is the most common line:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional"
Copy and paste above line just below it Add another string after proffessional. Add another parameter in new line after endquote as: viz. : /kernel=ntosboot.exe
Finally it would look like :
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional"
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional Hacked"/kernel=ntosboot.exe
Now when you want boot you will so 2 options like in multi-os. Select
the second one to get to the new bootscreen This way when you mess up you can always have the old version by selecting first option.
Now your problem, is the OS loading? I mean after 1/2 minute of blue bar is XP starting? I
If yes,Most probably what you have messed upon is that the guide must be on SP2 and you have non SP2 or vice versa. You may have to try different bitmap numbers. What you could do is download bootscreens available online (google it) and find out what numbers of bitmaps they have changed and change the same.
If OS is not loading then lord save you.
Whenever you change your bootscreen I would recommend using ntosboot.ext method as it's safest.