XP Pro SP3.
Somehow I have made a change and now my signature and all text in the mail message are at 6 point Arial rather than 12 point Times New Roman.
I know I can change each message, but it used to come up in 12 Pt. TNR.
Try: Tools -> Options -> Mail Format -> Stationery and Fonts Then ensure Stationery is set to , then click the "Font..." button and the check/change the options shown. Hopefully that will fix your problem.