USERNAME/PASSWORD

THERE IS NO CONSISTENT TIME OR DAY THE FOLLOWING HAPPENS.
When receiving email; the ENTER NETWORK PASSWORD box pops-up. My username is already shown there in the username field and my password is already there with a check-mark in the save this password Box. This may happen 1 time a day or every other time when I click on Receive Mail using MS OUTLOOK not express.
I have ran several virus scans from different antivirus software companies and found not 1 virus. The machine in question is running WINDOW XP Pro. HELP?