Outlook 03 asterisks and garbled address

Help! Just recently, when I send a message using Outlook 2003 my return address morphs into 11 asterisks, then the remaining characters. The person I e-mailed cannot reply, as the address is not recognized as a valid address. I’ve run Spysweeper, Windows Defender, and Zone Alarm Pro’s virus/spyware checker, and nothing shows up. I’ve deleted and re-enetered my e-mail account connection information in Outlook. I’ve run the pst folder repair program from Microsoft. Nothing works. I think it is in Outlook because I have a yahoo account, and if I e-mail from yahoo’s website, everything is fine.

Does anyone know what I could do to fix this?

Thanks in advance