The FAQ below comes from Metapad homepage -->
http://liquidninja.com/metapad/
But it works for all Notepad replacements.
Q: How do I replace notepad under Windows XP?
A: System File Protection is in full effect. This is what worked for me (but not many others):
Locate the filelist.xml file in the WINDOWS\system32\Restore folder and back up a copy of it. Open this file in Notepad and look in this section:
<Exclude>
<REC>%windir%\system.ini</REC>
<REC>%windir%\tasks\desktop.ini</REC>
<REC>%windir%\win.ini</REC>
<REC>*:\AUTOEXEC.BAT</REC>
<REC>*:\CONFIG.MSI</REC>
<REC>*:\CONFIG.SYS</REC>
</Exclude>
Add the line:
<REC>%windir%\notepad.exe</REC>
to the list of Excluded files. Now you should be able to overwrite notepad.exe.
You can also try the following to get around System File Protection. Be very quick or XP will replace the file and it won't work. Make sure you don't have the WinXP CD-ROM in your drive!
1) Copy metapad renamed as Notepad.exe to the clipboard.
2) Go to C:\WINDOWS\System32\dllcache
3) Paste the file there.
4) Go to C:\WINDOWS\System32 (using Backspace)
5) Paste the file there.
6) QUICKLY! Go back to C:\WINDOWS (another Backspace) and paste the file there.
If you have done this correctly, a message should pop up saying that a bad version of a Windows System file has been installed. Make sure you select 'Keep this file' or a similar option. If it doesn't work, keep trying, because it will. Just do the two pastes very fast.