On the game's store page, under "Minimum system requirements", a line says "The game is incompatible with netbook hardware". What does that mean? How do I determine whether my system is a netbook?

https://en.wikipedia.org/wiki/Netbook

Netbooks are very small notebooks, mostly powered by Intel Atom Processors.

Thanks; but that page is over 4200 words long, and a quick skim doesn't show anything that would clarify the system requirements of a particular 20-year-old game. Are you sure the answer is in there?

Just tell me the specs of your system. (CPU, GPU, RAM)

i5-8250U, Intel UHD Graphics 620, 8 GB.

The demo seems to run fine, apart from menu items flashing too fast and some keys not working.

That's definitely not netbook hardware. You can run the game with your system.

If you get the "DirectX 5 not found" error, read here how to solve it:
https://www.gog.com/forum/commandos_series/direct_x_5_not_found/post27

Thanks. I'll leave the topic open in hope of an official, general answer.

I ran it just fine on a laptop with incomparably weaker specs. You're fine.