Thanks, it looks like you may be right. I narrowed the issue down to a DDE communication problem. It seems that Internet Explorer is unable to contact mIRC via DDE, though I'm not sure why since Firefox is doing so without a problem.
To see whether this was an elevation issue, I disabled UAC and restarted Vista and tested again. This time Internet Explorer was able to contact mIRC via DDE and it all worked fine.
I turned UAC back on again and restarted Vista, and tested again and the problem was back. I then gave mIRC Administrator privileges, digitally signed it, and tested again. The problem was still there.
I then ran Internet Explorer as Adminstrator (right-click on Internet Explorer icon, select Run As Administrator) and tested again, and this time it worked fine with a plain mIRC 6.21.
It's possible this could be an Internet Explorer/Vista bug, or that mIRC isn't doing something right. I'll be looking into it further.