Vista - Using special characters on my tagline

Hi,

My son has a computer running XP Pro at his mums and Vista Ultimate at my
place. Both are running the same version of windows live messenger,
14.0.8064.206 (I think).

He is trying to use special characters in his tage line. Specifically a
double headed arrow which magnifies the following characters. He can enter
the arrow, and people can see the magnified icons on XP, Vista, and even
someone using Windows 7. But, on his XP computer, all he sees for the arrow
is a an empty square box (which means the character is untranslatable?). The
same problem occurs on his brothers XP computer at his mums, but both work
perfectly on their Vista computers at my place. Regardless of which computer
they are using, people the comminucate with see the desired result
(magnified emoticons).

I looked at regional settings, code pages, tried copying and pasting from
Character map. Nothing works.

Anyone know why the arrow is not displaying correctly on his XP machine?

thanks