The code works OK with a simple example I created here using Word 2003, so
my best guess is that when the code gets to this point, the ActiveDocument
is not the one it should be:

' Reprotect the main mail merge document.
ActiveDocument.Protect Password:="", NoReset:=True, _
  Type:=wdAllowOnlyFormFields

However, it's difficult to see why that might be so. There are a couple of
things ytou could do to investigate:
a. learn how to use the debug facilities within VBA (breakpoints, stepping
through code etc. with the Debug toolbar)
b. put msgbox or debug.print statements into the code to examine the values
of the variables involved. msgbox makes a message box pop up, whereas
debug.print displays the results in the Immediate Window (in the VBA
editor), which you can enable using ctrl-G in the VBA Editor.

e.g. I might start with the following:

put

MsgBox "swindowmain:" & sWindowMain

before

' Reactivate the main merge document.
Windows(sWindowMain).Activate

and put

MsgBox "activedocument: " & ActiveDocument.Name

before

' Reprotect the main mail merge document.
ActiveDocument.Protect Password:="", NoReset:=True, _
  Type:=wdAllowOnlyFormFields

and see what results you get.

On the subject of spell checking, I think you have to play a similar game,
i.e. use a macro to unprotect the document, spell check, then reprotect, and
make that macro available to your users by associating it with a toolbar
button, a menu option, or a keyboard shortcut, e.g. in Tools|Customize.
However, the macro really has to be either in the normal.dot template, or if
your mail merge main document is attached to another template, put it in
that template. Putting it in the mail merge main document itself will not
work.

You could use a macro like

Sub mySpellCheck()

With ActiveDocument
 If .ProtectionType <> wdNoProtection Then
   .Unprotect
 End If
 .CheckSpelling
 .Protect Password:="", NoReset:=True, _
  Type:=wdAllowOnlyFormFields
End With

End Sub

Peter Jamieson