How can I replace core system files with WindDbg?

I want to replace Windows core system files with the new one I receivced from MS for test.
And Microsoft staff inform me the way to replace core system files like belows.

1.attach the machine to a kernel debugger
2.add the new *.sys files to %windir%\driver cache\i386
3.then overwrite the test drivers on th %windir%\system32\drivers

That way Windows file protection (WFP) will not replace the *.sys files from the CAB in driver cache. Remember that you should keep the kernel debugger running when you want to do step #3.

On my XP Media Center Edition,
1st: I insert ‘/debug’ option to boot.ini file.
2nd: I exececuted WinDbg and selected ‘Kernel Debug…’ from File menu.
3rd: From popup window, I selected ‘Kernel debugging of the local machine’
4th: And copied *.sys files to ‘Windows\driver cache\i386’ folder
5th: And also copied *.sys files to ‘Windows\system32\drivers’ folder

But After rebooting, system files replaced to the original one.

Please tell me what’s the problem.

Thanks in advance.