Job done in about ten minutes!
She only wanted Word 2003 but I recommended Excel 2003 as well, since she uses Excel a fair bit. Installation took under five minutes (amazing!)
Word 2003 started immediately, and the clever AutoText utilities she'd written worked straight away.
Starting Word 2007 brought up the "Updating...." screen. Once that had completed, the one-line fix told the machine not to do that any more. *
There are now two toolbar icons for Word and Excel, and they bring up 2003 or 2007 as required.
Easy.
* The one-line fix, to stop MS Office doing an installation update every time the "other" version is started, is this, typed as a RUN command:
reg add HKCU\Software\Microsoft\Office\11.0\Word\Options /v NoReReg /t REG_DWORD /d 1