You are going to be trapped in a room for 12 hours with a mid 2000s office desktop with no internet connection and an external hard drive; what are you putting on the hard drive?
I've got some DOOM WADs I have been meaning to play so I would probably grab Trench Foot, Total Chaos, and the sequel to Ashes 2063, Ashes: Afterglow with a portable install of GZDoom to play them.
After that I'd probably bring Star Trek TOS and a MOBI copy of Neuromancer by William Gibson combined with a portable install of VLC and Calibre in case the computer didn't have applications that support the file format.
What about you?
I wanted to phrase this in a way where it isn't a prolonged or desert island style question where the responsible idea would be to bring Wikipedia ZIMs and educational PDFs. It's just an awkward amount of time to kill. The mid 2000s office desktop stipulation is just an additional challenge so you can't just bring in a copy of Baldur's Gate 3 or Cyberpunk 2077.
Edit: By mid 2000s I meant around 2005; the XP or Vista.
Some retro computer emulators(Atari 8-bit, C64) and my dev environment for them - when you target old stuff you can customize the whole dev tooling setup with very little compromise, especially if you go the route of assembly/Basic/Forth and then pile on higher level build steps. I'd have to be careful around the potential problem of "whoops there's a 64-bit binary in there and I'm on a 32-bit OS".
Basically if I were back in college it'd be that all the time, and then VLC and some anime or movies in 480p. No sense in keeping up with those darn 2000's games.