Word 2003 windows temporarily frozen

I have the following nuisance problem with Word 2003 under XP Home. If the
focus is taken away from a Word window, e.g., to look at another Word window
or any other open window, then when clicking back to it this Word window it
is frozen for 10-15 seconds: even the cursor doesn't blink. Hour glass
shows. The same thing happens when the file is first opened. It does NOT say
'virus checking' during this time. Even the Close box does not work until
after this delay. It even happens with a totally blank document. It does NOT
happen when a child dialog box, e.g., Save as, closes. It is NOT HD
thrashing. I have 2Gb RAM.

How can I find out what is causing this delay?

Derek