I've been going over this one through out the day and here is what I've gotten since. First I started digging around the Radiant folder in Application Data. There was a folder called wolf.game. So I copied it, renamed the copy to oa.game and set everything up for OA. Then I changed global.pref so that everything pointed to OA. Note that this was after uninstalling Radiant and resetting all of the stuff in Radiant's game folder (and re-adding the oa.game folder). As soon as I restarted GTKRadiant reset everything back to the old wolf settings.
Just to make sure that I wasn't running some kind of modified version of Radiant, I clean everything out again, download and install another copy of GTKRadiant from
http://www.qeradiant.com/cgi-bin/trac.cgi. Same thing.
At this point I'm not in a good mood. So I uninstall GTKRadiant, clean everything out again in Program Files and Application Data, restart, backup the registry and delete or modify everything that has GTKRadiant related to it. I restart, douse a little holy water on my machine and reinstall the new copy of Radiant. It asks me to find the engine path for RTWC:ET again.
I huddled up in the corner and resisted all urges.
At this point I take it that I should assume that the problem is Windows 2000?