Mike, I had a similar issue at the time but as I'm prone to computers falling over (it's my animal magnetism, I'm sure) I thought it was just me. It probably was, but... What I did in the end (FIRST backing up all songs that were in the Notation folder) was uninstall the lot, then new-install GPO studio and GPO, which I had to do a couple of times to get the multis up (a known installation problem, see the Garritan website where there turns out to be a simpler fix which, obviously, I didn't look for until I no longer needed it).

Once GPO AND Garritan Studio were installed and working, I new-installed Composer which opened without problems, played music but made no sound. At that stage I went into the MIDI devices quick setup and made sure this tiem I'd setup Garritan Studio 1, Studio 2 etc as devices, THEN it all worked.

I have no idea why GPO didn't properly install, or why Composer may or may not have taken offence. I do know that it took me five attempts to install MS Office 2000 (full legal professional version)when I last did it, so that it worked properly on my systems, (and there were still odd drivers missing....they were on the cd, just didn't install).

I just work on the basis that once a program is huge, which is especially the case for GPO, things don't always read or get read. Or alternatively, computers often do things for no apparent reason, a belief firmly established in 1968 and which I've never had reason to shift from. Mark or the more technically minded might have a better fix, but I'd just try again.