1. Open following folder:
You can open it by typing the path in RUN dialog box and press Enter.
2. Now take ownership of "wordpad.exe" file. To take ownership, please follow this tutorial:
Add Take Ownership Option in File / Folder Context Menu in Windows Vista and 7
3. Now rename the "wordpad.exe" file to "wordpad_backup.exe".
4. Now go to "en-US" folder and take ownership of "wordpad.exe.mui" file. After taking ownership rename the file to "wordpad_backup.exe.mui".
5. Now copy "wordpad.exe" file from Vista and paste it in "%Programfiles%\Windows NT\Accessories" folder.
Do the same thing for "wordpad.exe.mui" file, copy it from Vista and paste in "%Programfiles%\Windows NT\Accessories\en-US" folder.
That's it. Now Windows 7 will start using the old Wordpad.