Yup, it's this one:
http://worms2d.info/Troubleshooting_FAQ#.22ERROR:_Could_Not_open.2Fcre ate_team_File.22_when_editing_teams
I use Windows XP SP 3
Funny thing is, I have Windows installed on D:, instead of on C:
But I can't run it as administrator, even when I'm the only user of this computer! Some funny error message appeared, I have uninstalled WA now so I can't tell you what's the matter. I tried to install WA on both hard disk drives, even in "all users" folder on D: drive.
I tried creating WG.WGT manually (using notepad)(it allowed me to create it in Teams folder) and then editing it with either game and TeamED, both were unsuccesful.
I ran out of ideas.
Help?

I'd be interested in knowing what that error message was. I suspect it is not caused by W:A itself.

Here is how to give unprivileged users write access to the W:A directory.
If you use a non-English version of Windows, text referred in these instructions may be appropriately localized.


Browse to the parent directory of the directory you installed W:A in.
Right click the Worms Armageddon directory, select Properties.
Switch to the Security tab.
If the Security tab is not present, you will have to configure Windows to display it. There are some tutorials on the web (http://www.google.com/search?q=restore+vista+security+tab).
If an Edit button is present under the first list box, click it and confirm the UAC prompt - otherwise, skip this step.
Select the Users item in the first list box.
In the second list box, click the check box under Allow on the first row (Full control).
Click OK to dismiss the check box.

I would hug you, but you'd have to buy me bus ticket :D
It works now :D