Hi nekto,
If jpgs are opened by some applications there is good news: files are not damaged in fact, and problem laying in operating system itself. It is hard to say what is exact reason for all this without access to the machine, but I'll rather be suspicious in terms of system upgrades before AD was installed. A *.dll file is Dynamic Linking Library, linked to actual code if needed to do specific task (in very simple words), and this linking process may be affected by some system errors. I had similar issue a good few years ago with Windows Vista and only working solution was format the system drive and clean re-installation of the system. And no, there was no virus or other nasty stuff in the machine - just old and well known ability of the Windows family OS' to accumulate errors in vast space of system registers. To be honest - since that happen, a yearly refreshing of the Windows became my natural habit, and I thing that MS is aware about the problem because we can see more and more advanced 'refreshment' tools in Windows since Vista.
Probably there is some way to fix the problem without system being reinstalled, like *.dll files replacement for the new ones extracted from installation disk, registry scan etc, but as Linux, OSX and Windows user I have experience that sometimes setting things up as new is the best way in case of MS operating system... You'll lost few hours, obviously, but maybe this is the best way...