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.
For anyone above a certain age with kids below a certain age this isn't a punishment or a challenge, this is A DREAM!!
Heroes of might and magic 3, system shock 2, Anno (any version up to 1503), Morrowind, Civilization 3, Age of wonders shadow magic, Baldur's gate 1 and 2...
Whenever my wife finds the time she goes to her room to play Morrowind. She just got a new laptop and the first thing I did was install OpenMW and copy her save file.
It goes without saying that you couldn't finish any of those games in those 12 hours, except for System Shock 2.