I finally figured out how to play Hidden & Dangerous 1 with my friends on operating systems newer than Windows XP. You and your friends have to be in the same network for this to work:

Step 1:
Turn DirectPlay on. To do this, you have to search for “Turn Windows features on or off” with the help of your start menu. Open the folder “Legacy Components” and check “DirectPlay”. Click “OK” and restart your computer.

Step 2:
Deactivate your Windows Firewall.

Step 3:
1. Host:
Start “Hidden & Dangerous: Action Pack” (Deluxe doesn’t work), select “Network Game” and choose “Setting”. Connection has to be “TCP/.IP”. Create your player name. Now click Escape, choose “Connect” and select “Host Game”. Create a game profile.

2. Join:
Start “Hidden & Dangerous: Action Pack” (Deluxe doesn’t work), select “Network Game” and choose “Setting”. Connection has to be “TCP/.IP”. Select “Data” and fill in the IP of your host (you don’t have to fill in the port). Create your player name (don’t use the same name as your host). Now click Escape, choose “Connect” and join the game of your host.

Step 4:
Now the host can start the game.