Is there any practical way to secure access to one’s handwriting font so that it cannot be used by someone else? (WinXP)
The only reliable way would be to install it only while you are using the PC.
Hiding the signature in the Private Use Area will not hide it even from the casual user, since it will appear on the Insert Symbol menu. The best one could do I think is to assign it to the Private Use area in a standard font that few people use (like Lucida Sans Unicode) or Marlet. You could type it with Alter + codepoint as one can type fi with Alter + 64257. If you add it to Marlet, which is a symbol font, you could assign it to 61562 and type it with the standard keyboard using lowercase “z” for example.