Davack,
Although I haven't upgraded to 2007 myself, I have experienced upgrade and
new install pain. I assume that 2007 may use some the same folders,
filenames, and /or the same files for many of the supporting files.
<filename>.dll.
Even if you are careful, some 2003 functions may not work after the 2007
install, because <filename>.dll for 2003 was in version 4.5, and
<filename>.dll for 2007 is in version 6.5. Word (Office 2003) will not be
able to understand version 6.5.
You might have better luck if your hard drive is partitioned into different
bootable partitions. You could then install 2007 to a separate partition
leaving the original bootable partition alone. Alternately, if word 2007 is
installed to hard drive d while word 2003 is left on drive c, could also
mitigate some issues. None of these solutions is 100%. Before doing your
upgrade, do a full backup.
Also, be prepared to teach your spouse 2007, or to learn to love 2003.