Hi,

I've bought Stronghold Crusader HD, after installing the game, it always starts Stronghold Crusader Extreme.

How do I play the normal Crusader in HD but without the Extreme part?

GOG did this wrong. They sell Crusader and Extreme in one but call the Extreme version the HD version (Extreme is also HD but a completely different game).
People have stated this many times, now 1-2 years later it's still wrong. >.<

You can launch it in Galaxy through the More -> Other menu. I know, it's terrible.

I assume you used the offline ("classic") installer, not the Galaxy one.
The installer creates shortcuts only to the SC: Extreme exe. In order to start HD version of original stronghold you need to navigate to the installation folder (default C:\GOG Games\Stronghold Crusader Extreme HD) and start Stronghold Crusader.exe

2019 UPDATE: The problem, clearly, is using the "Galaxy" installer. So uninstall the whole mess. Then start again.

This time, instead of installing from the oh-so-tempting big blue "install" button, look on the right-hand side of the game's description screen. You should see a list of "EXTRAS". Near the bottom of the list is "Classic Stronghold Crusader". Click that extra and it will install the original the old-fashioned way. No need to tinker with workarounds beyond that.

This is wrong, you'll just be installing Stronghold Crusader + Extreme version 1.2.1 instead of 1.41.