I’d wager you’re not doing much with that machine - in my experience '98 required an annnual re-install, and I recently had to reformat even my Win7 box after a C++ library version mismatch hosed a few very specific programs. (And I pulled out all the stops on that - used to do XP phone support for Microsoft).
For a friend’s machine, I’m unlikely to go spelunking in the registry or troubleshoot SxS issues, and after quizzing them on what data they actually care about, they would easily be left with stosh’s question.
On the above note, just to be safe: stosh, everything means everything, including saved passwords, installed programs, Microsoft Office install keys, emails, photos, mp3s, games, game saves, game high-scores, etc.