The problem is that the header of the memo file is probably invalid. A reason for this may be a not properly Windows shutdown, or just a problem with your filesystem.
I can recommand the stonefield database toolkit (www.stonefield.com) which provides functionality to detect this problem and is able to fix the problem.

Leo
>Every so often this error appears during the execution of a USE command.
>
>The unusual thing is it doesn't happen everytime.
>
>The Memo file is present and OK.