My fonts in yahoo messenger keep changing by them self to a very small font o/s is vista. Help please?

Hi I just got a new system and it has vista installed on it,. not evertime i log into yahoo messenger i chat for a littel then the fonts change to a font that is so small you cant see. I change the font back and a few min later it changes back again. i have tried to close the program and start it again and still get the same results. does anyone have any idea what is going on and maybe how to fix it........PLEASE I NEED HELP. I can not continue this wayMy fonts in yahoo messenger keep changing by them self to a very small font o/s is vista. Help please?
To keep your fonts as you like for every chat

open messenger - on top toolbar %26gt; messenger %26gt; preferences %26gt; appearance%26gt; select fonts and colours you want %26gt; apply %26gt; ok %26gt;

sign out of messenger

sign back in

done !!