I have a dual screen setup, one monitor being quite large, the other quite tiny.
Now i had a chance at first launch after install to pick the screen i wanted, however i made a mistake and picked the wrong one. :/
Problem is i don't find how to get back that screen, i don't find any option in the ini files that could refer to it and reïnstalling didn't give me a new opportunity to set the screen.
So what i'm looking for is a way to erase really everything of this game, in order to get the opportunity to pick screen again after install. Or a way to change it after installation and first run. :x

No problem, have fun enjoying this amazing game.

I did the same thing, but the solution didn't help for me. Reinstalling, deleting anything related to The Guild in regedit. Nothing worked. I did manage to set the game to play in windowed mode though, which is good enough for me.

To set it to windowed mode, go to the game folder and edit game.ini

Change:
Bildmodus=FULLSCREEN

To:
Bildmodus=DIRECTWINDOW

When you launch the game, you should now play in windowed mode. As a bonus, if you have a widescreen monitor, you may have less mouse stuttering.

The real solution is this:

1. Open regedit
2. HKEY_CURRENT_USER->Software
3. Delete the "Ahead Entertainment" Key and its subkeys (the folder)
4. Start game and choose again.

This is AFAIK the ONLY way to do this.

I found the same solution that Kazper tells, using Registry snapshots comparision - it works.

But I wish to add as welll, that there is some bug in the _TnL version, or talking more precisely in d8_resource.dll library it show a monitors selection list on startup populated with the same (second) monitor. So there is no mater what yuou select, it anyway runs on the single monitor, and you could not change it.

Of course it would be nice if some one handle this issue and fix this dll,