Microsoft Windows XP Professional "set email address myself" Questions

...a valid email address in its own right. I could be wrong, but as I just set up a Yahoo account and myself, and all my friends have a similar setup but the email addresses are used for messenger

...email is ISP based and received via Outlook Express, I use Windows XP> I have changed no settings that I know of from before the issue began, so its a mystery to me. I have even sent myself mail from ...