Hi Mike-
Well, well- let's blow the dust off this one, shall we?

This was a fun app to build, especially after Helmut Steel provided some adult supervision on the DOS side of it. It looks like that bit still works fine after nearly 10 years, but the Adobe side has changed.
Acrobat is doing the right thing. The ability to have a PDF file launch an executable was a handy, but dangerous feature. The batch files it is trying to run to change flyable helos could just as easily contain commands to format your HD. Lovely.
So while it should be possible to modify the security settings in Adobe to allow for this all to still work within one specific folder, (Under Edit>Preferences>Security (Enhanced) and Trust Manager), it doesn't actually seem to work. (Unless I need to reboot- didn't try that.) So I'm getting the same error as you, and I am also a local admin. I suspect there is a MS change at work in the operating system.
Hmmm.
It probably isn't worth anyone spending too much time on this. The file swapping approach was used prior to us figuring out how to edit the actual EXE. (And even that approach has advanced light years since my day thanks to Samovar, PSYKLIK, Will Baldwin et. al.) But you can still use ModM8 if you don't mind monkeying around in the basement a bit, but be very cautious if you have other, later GS mods installed. Some of the file names may have changed. Can't remember, to be honest. Make a clean install for ModM8 if you are not sure.
Today's problem is in the User Interface - the Adobe Acrobat file security settings. But all that PDF does is give you a handy way to run individual DOS Batch files in the background to move files around in specific ways. Those files still work, but you'll need to run them manually. Here's how:
ModM8 installs in your Gunship! folder in a new subfolder named "mods". In my case, it is here: C:\Program Files\Microprose\Gunship!\mods. The most important file of all is there in the mods folder - it is called "resetall.bat. Running this file at any time puts everything back where it was before you installed ModM8.
In subfolders under the "mods" folder are all the individual changes you can make. Inside each one is a batch file, such C:\Program Files\Microprose\Gunship!\mods\uh60\uh60.bat . When you run that file, it moves things around or renames them so that the next time you fly a mission involving the Eurocopter Tiger, you fly a Blackhawk instead. Crude. but effective.
The terrains are done the same way. If you execute the mods/ter_win/ter_win.bat file, you get a winter wonderland to fly about in. When your feet get too cold, exit and run the mods/ter_orig/ter_orig.bat file to return to the standard terrain.
Hope some of that helps. I just happened to swing by SimHQ today, but it may be a while before I can check back. Glad you are having fun with GS. Still a remarkable bit of work after all these years.
fng
edit:
I forgot something: if you are using the Winter or Desert terrains, I suggest one other mod: The HiDef terrain executable. Makes a huge difference in how good things look. This is just a new version of the standard, patched NO-CD exe, but with an interesting modification: It only uses the high detail 3D objects at all distances.
http://www.simwarrior.com/gunship/utilities/GSHiD_v1.zipJust unzip it into your Gunship! folder next to the original gunship.exe, and make a shortcut for it if you like. Works well in the normal terrain, too, as you can see here:
http://www.youtube.com/watch?v=_m79Q5bQ4eE