SP2 upgrade frozen - what to do?

Upgrading my wife's XP Home PC to SP2 with WU. Downloaded proceeded
OK, about 4 hours on dial-up. But it has now been apparently stuck in
the installation stage for about half an hour, with no progress bar
movement (only 3 green 'bars', which is same as I found it after the
download) and no HD activity. Messages read
"Downloading SP2... done!"
"Initializing installation...done!"
"Installing SP2..." (This is where it is frozen.)

Before clicking Cancel (the only option offered), I'm posting here to
see if anyone can offer any other suggestions on how to proceed
please.

FWIW, all other applications that can be closed *are* closed. NAV is
in 'Autoprotect disabled'. I cannot completely close it, even from TM.

--
Terry, West Sussex, UK