I had the same problem on my machine when I tried to install SP2. Turned out that it was my boot screen changer that caused the problem, for it used the old SP1 ntoskrnl.exe file version, not the new SP2 one.
Your problem may be because of something else, but it is very easily fixed. You can simply uninstall SP2 from your XP machine to get it working again. You can approach this in two ways:
The first is uninstalling it in safe mode, from add or remove programs, or by going into run in safe mode and running the uninstallation program: c:\windows\$NtServicePackUninstall$\spuninst\spuni nst.exe
However, if you can't get into safe mode because your computer boots somewhere during the loading process, you can uninstall SP2 within the command line interface of the Windows XP Recovery Console. You'll need your XP CD for this, or simply access it on your machine if it is installed on your hard drive. At the command prompt, type: cd $ntservicepackuninstall$\spuninst
then enter. Once you've changed directories, type: batch spuninst.txt
You can only perform an uninstallation if you have retained the backup SP1 files on your Hard Drive. The complete methods in greater detail are available here: