Thanks. Haven't tested it a lot yet but on the first try I renamed the file, saved it, then exited Streets & Trips. I went to the Windows Explorer, deleted the original file, and renamed the (renamed) file back to my original name.
When Streets & Trips opened it it was to the correct map, not Coco again.
However, I really can't be doing this every time I save the map. Is there a "config" file or program that will change Streets & Trips default behavior? This was not happening previously, it just started a month or so ago. I had installed the trial offer of Streets & Trips 2008 about a month ago, didn't like it, and removed it the same day. I wonder if that left something behind to cause this. None of my other computers running Streets & Trips 2006 have ever behaved like this.