Used XP SP3 mode since Vista days.

Never use Vsync. (fps limited to 60 in vid driver settings; without limiting this system gives 150-300fps depending on track and number of cars; the only slowdowns are when navigating the menus in the default install)

Only time I've used windowed mode was ages ago when someone posted it might improve fps in flightsim; results were inconclusive on my system.

processes running in the background?!
This is Win11, it requires two dozen processes to open a text file.

But all three N2k3 installs run fine once on the track, it's dealing with this every time I start them -

With the two GTP installs I "OK" this and they run fine. With the default install I must immediately click on a menu option after "OK" or the game shuts down, when it is running all menu navigation is ridiculously slow.

Now I get it....

Judging by that error prompt, it appears that the main NR2003 folder has some files that went bad during said transfer procedure. -Not sure 'bout them secondary installs though....-

[It definitely helps running the game's installer in Repair mode and then the update patch before reapplying the v1.2.0.1 No-CD for Windows 10 and the Large Address Aware Patch, just to be on the safe side of things.
(Better make a backup of all the Players, Series & Tracks folders beforehand #JustInCase)

[Once finished, reapply said folders and run the Graphics Configuration Tool with your preffered settings.

-Drivers....to your cars....