Right, so I've installed BGT because of problems playing the vanilla version (I became unable to talk to certain characters even with fixes). Everything seems to be working really well, although I'm not up to the point I started having trouble.
Anyway, I'm having an odd non-game breaking issue, and searching found a similar issue for the Enhanced version of BG, but I didn't see anything on BGT. What happens is that I go to edit a map marker and save it. It takes a long time to accept the marker, but eventually it does. Then, the next time I go to save it crashes out with the error:

Runtime Error!
D:\My Games\Baldur's Gate 2\BGMain.exe
abnormal program termination

I've done a complete clear and reinstall with no luck. Any ideas?

When you install BGT you *absolutely must* install with an unmodded (including fixes) BG1. BGT requires this, because it uses BG1 resources, and will crap out somewhere along the line if it is not done cleanly.

So, your best bet is to uninstall both BG1 and BG2 (back up your save games, or leave them in place, but be aware of compatibility issues with save games, like if you use the Widecreen mod, for example). Once uninstalled, make sure you clean out (delete) anything in the \temp\ and \override\ folders, in fact delete ALL folders except for your \save\ folders. When you have done that, reinstall BG1 and leave it as it is!

Now, reinstall BG2 and install Ascension and Turnabout first, and in that order, if you use them.

Next, install the BG2 Fix Pack.

Now you can install BGT, then any other mods afterwards.