Mac Mail Arial text displays as other fonts on PCs

[Queen Mojaji]

I've tried everything and asked everyone and I really hope someone out there can help me! I'm using Mac Mail, OS 10.5.2. and want all email text to be read by recipients in the same style that I set it in on my Mac. This happens occasionally, but with most PC users my Arial 10pt email body text displays in Times Roman 12 pt. Horror of horrors. My preferences are set to rich text (on advice of mac person of skill) all I can think of is that my Arial system font is out of sync with the recipients; or that I need to move to Entourage, which can't be right??!!! I'd be most grateful if someone with knowledge of this could help in any way at all.