You can download Notepad2 from http://www.flos-freeware.ch/notepad2.html

Step 1)

Find c:\windows\notepad.exe and c:\windows\system32\notepad.exe (use Windows Explorer)

Change the owner of both files to “Administrators”
In case you are unsure…

Right click on notepad.exe, select properties, select the “Security” panel.
Click “Advanced”

In the resulting dialog, select the Owner tab, then click “Edit…”

Highlight “Administrators” and then click Ok.

Then, back in the Security panel (from start of step 1) click on the Edit button (below list of groups)

Select the Administrators group (see bottom left image below) then click the Full control checkbox, which should select all the other checkboxes.
Click ok.

Step 2)

Rename notepad2.exe to notepad.exe.
Fairly self explanatory – rename wherever you have unzipped notepad2.zip

Step 3)

Move the newly named notepad.exe into c:\windows and c:\windows\system32
Replace the existing notepad.exe with the new one.

Done

Windows will now use notepad2 as it would of used standard notepad.