[plt-scheme] Cannot install 207 on Windows

To explain the situation:

 1) The problem is almost certainly a bug in MrEd, which for some
    reason is exposed by the new (as of 206) install process.

 2) I have no idea what the bug might be, but it seems to depend on the
    timing of something. Perhaps it's a race condition.

 3) I have never been able to provoke a bad install myself, which makes
    it very difficult for me to figure out where the bug might be.

So, all of this information on the problem is useful. If someone
manages to get a stack trace at the time of a crash, that would be
extremely useful.

Thanks,
Matthew