I've recently set up a windows xp machine and so far nearly all of the gog games are played without a problem. But for this game, I have the serial key problem.

The Regs.cmd doesn't work in WinXP, says the commands are invalid several times. Tried typing a lot of serial keys including my own serial key GOG gave me.

The problem is with the regs.cmd, I think. the command codes are not just written for XP, I guess.

Anyone have any solution to this?

Thanks in advance!

I have the same problem...I see there is no solution, what? :-(

I experienced the same problem on my Win XP Retro PC. I wasn't able to find a fix. It looks like GOG broke WinXP while supporting newer OS versions of WIndows. I read this in some forum posts concerning other games facing the same problem.
I can only tell you that the GOG Version of S3 works fine with Windows 7. I can also bring it to life on Fedora 44 using Lutris. I suggest you grab an original copy of Settler III on eBay and play it this way. The GOG version has no widescreen support anyway so you don't miss out any important updates except the need for a disc drive.

if serial codes depleted, ask GOG for new, not copy from forum. Distribution bug of GOG or your keys, but options what to do missing when changed approach to play protected games.

Ubisoft make new releases, but does not solve this bug outside of keys, recompile needed.

Some games(like Age of Empires 2 HD) make engine upgrade to make it easier for lower grade PC - but in terms of shader level - often choice how to do that, but automation could destroy able to play and experience deliver. And when additional techs approaching into game, Engine Eras - fallbacks, shims and settings level be missing, maybe an option for this XP points and upgrade/downgrade(sw collection) delivery.

Compatibility Windows 8 mode + Run as Administrator could help, but that is weird for some thoughts outside of levelships when click and plug is an option to make, detection of installer is somehow prohibited under arrest of Delivery & Recompile 4 GOG, not only. With classic tag. HD Way now.

If uncertain, check GH for patches, but Ubisoft has newer releases than here on GOG, accounts and delivery could be missing on markets, publisher choice is uncertain, french academy for movies and by movies, in cinemas in Europe and distros, on the wild elsewhere.