But stock version of Windows has a ton of stuff you can use. All sorts of graphics and audio files. And that again depends on which version you're basing it on.
I think it would be cool to run a competition with more specific (and platform independent) set of constraints. I guess I should spend more time thinking about how to organise it myself than complaining that no-one else has!
It's usually forbidden to rely on those files as they can disappear with updates of windows. The best example I can think of is General MIDI. The files were avaialble with XP and below, and is now often explicitly forbidden because it's not available anymore or not in the same form making demos incompatible.
It's also often forbidden to use the filename to store data. There was that case of the 256B demo that relied on a deep hierarchy of directories to work :)
Indeed, the rules for the Revision party where this was released require the intro to run on Windows 10 (so implicitly no MIDI) and specify that the sample music that comes with Windows will be deleted.
In reality, the multi-gigabyte OS everyone seems to complain about when a sizecoding demoscene production is shown is mostly there as a compatibility layer. You simply can't use modern GPUs without it. One could conceivably do all that on the bare metal with not that much difference in size, the problem is that there is no powerful enough bare metal platform where you can do that.
If you prefer to work closer to the hardware, that's what the "oldschool" and "wild" categories are for. But these are more about overcoming the limited abilities of these platforms than pure sizecoding.
There are competitions for all sorts of retro computer and game console platforms which are "purer" in the sense that there's little to no operating system at all, so the demo has to be programmed against the bare metal. That's not feasible for modern PCs though, you need the OS infrastructure and drivers to abstract over the variety of hardware.
Or if you're web-inclined you could use the browser as your OS:
Has been done and discussed (flamed) ages ago, in the early Windows demos era. An operating system has a lot of “free” data, graphical or otherwise (from wallpapers to icon resources), which can be used directly or processed into something else. The code to access them cam become too big, but hacks are possible. There's also a question of fonts. Is it OK to use them to “just put some text on screen”, is it OK to use them as a source of all kinds of curves? It is hard to define where “the program” ends, and the rest of the system starts. After all, they all need to load libraries to interact with GPU into their process space. Those libraries have debug functions, example data, and other junk which is not used to put pictures on screen, but is available. Then what if a tiny application links to 50 system libraries only to have a database of potentially suitable data sequences here and there?
And long before that, even regular software on microcomputers struggling to save each byte relied on known values being in known locations in ROM. It worked, because each device came from the factory with the same firmware, and it could never ever be changed for the reason I've just mentioned.
The solution turns out to be pretty simple: if you think you're smart, other smart people will study your trick, and decide whether it's impressive, or just a one-time joke. Formal rules for demoparties don't mention many possible “size extending techniques” because it's generally accepted that they won't help much compared to what can be done in the same amount of bytes directly by a competent author.
I think it would be cool to run a competition with more specific (and platform independent) set of constraints. I guess I should spend more time thinking about how to organise it myself than complaining that no-one else has!