WIA (Windows Imaging something) - why such a stopper?

My friend's XP computer was very hard to use: clicking on My Computer in
Windows Explorer caused a sort of freeze, as did using the Send To menu,
DIMAGE (software that came with a camera) popped up a "Server Busy"
window (finally opening after about 15 minutes), and save and load
windows, if you used the up button or the drop-down list to change the
save/load location from the default, also locked up. Bringing up the
TWAIN selection list also gave a freeze or _long_ pause, though if you
didn't change the selection but just used something (well, we tried the
scanner), it worked fine.

Googling brought the fact that lots of people had similar problems, and
a few reported that turning off the "service" WIA made an amazing
difference.

WIA seems to be part of Windows, and something to do with getting images
from cameras and scanners. In my friend's it case wasn't running, but
showed as "Starting" - this being some time after the computer had been
running.

We set it to Disabled (though not being sure what the difference between
Disabled and Manual would be; the other option, which it was, being
Automatic), and restarted the computer.

Amazing; everything is back as it should be - all the things that had
either frozen or paused for some minutes now work fast enough. The
scanner still works (Canon), the film scanner ditto (and we can select
which of them via the TWAIN selection list, which comes up at once). We
haven't tried anything that relies on Windows alone, such as a webcam (I
suspect that _wouldn't_ work).

What I'm curious about is: why does this "service", which seems to be a
standard part of Microsoft Windows, cause such apparently unrelated
problems when started (or rather allowed to try to start)? [And, for
that matter, what triggered the slowdown: probably something enabled the
service, such as a Windows update, but it could have been something else
which interacted badly with WIA?]
--
J. P. Gilliver. UMRA: 1960/<1985 MB++G.5AL-IS-P--Ch++(p)Ar@T0H+Sh0!:`)DNAf

If vegetarians eat vegetables,..beware of humanitarians!