Hey,

Just got worms united, the main menu opens up no problems at all. However whenever I try and open any of the sub menus they instantly return to the main menu again. I'm on windows 7 (64bit) and have already set the game to run in windows 2000 mode but that hasn't helped.

Any ideas?

It's a bug. I had the same problem when I played this game in the 90s on my Windows 95 computer. It's not specific to the GOG release or Dosbox. The bug description is as follows:

Start the game and click "play game". It will open the select team menu and scroll right back to the main menu. Also, random button presses will sometimes occur in the menus and the game, such as randomly selecting menu items and weapons.

The bug is totally random and the only thing that helps is to quit the game and start it again. You may have to quit and restart multiple times until it disappears, sometimes 5 or 10 times. Once you start the game, click "play game" and the menu stays open, it started without the bug and it'll work fine until the next start.