After my recent experiences, I think I'll give up!
As I said, I wanted to join a soundless rotated avi clip and its separated wav sound file. MS Movie Maker succesfully joined the two together, but it converted it from portrait to landscape mode (without rotating it), which means that the people in it now appeared as dumpy dwarfs! Despite the fact that they were now the right way up and speaking it wasn't quite what I wanted!
More worryingly, I found out that MS Explorer was now partially crippled. Now this may have been because I'd also just downloaded the latest batch of MS security patches because of the W32Blaster worm scare. In any event, I used the XP System Restore to get me back to the point before I loaded Movie Maker, which seemed to solve the problem.