Just to update: a run of Combofix on the machine found a half-dozen issues, but resolving these hasn't solved the original problem. A cursory glance showed none of these would have been related to Reader anyway.
I also need to clarify, in my original post I said "A" and "G" would not work; that's not strictly true. Capital "A" and "G" work just fine; "a" and "g" do not.
Message was edited by: matt.beamish
One of our staff has attempted to both rebuild the font folder and reinstall all the Windows fonts to see if this solved the issue, to no avail.
Our technician has solved the problem. He ran the Windows Installer Removal Tool (msicuu) and removed all items pertaining to Adobe Reader; he also removed all Adobe folders and registry entries. Finally, he reinstalled Adobe Reader 9.3 and found the problem had been resolved.
I hope this helps someone.
Thanks for coming back and posting your solution.