Thanks for the advice.
I've done this on the standard template and it still reverts to TNR font.
Secondly, on the email template from Outlook, there is no facility to select a default font - at least none that I can see. Futhermore, it starts off with Ariel and suddenly changes to TNR as I start typing - apparently at random. Plus it changes part (but not all) the sig - all the numbers change to TNR and the rest stay in Ariel.
I found the problem. There's a conflict with Adobe Reader 7.0 and XP which makes this happen.
Here's the solutions http://www.adobe.com/support/techdocs/331259.html