Close down Outlook Express and don't open it again until you are satisfied you have the problem licked. If you do need to open OE to check on something, disconnect the PC from the Net first.
I suggest you use another antivirus check. Don't just use the AVG one. A good free online, current one is available at
www.pandasoftware.com. If you do find anything, make sure you delete it from the recycle bin, and reboot the PC before doing anything else to minimise risks of it reinfecting.
AVG is OK as far as it goes, but it is free. Can you really afford to trust your work PC to somethign you get for nothing. There is a reason it's free, it doesn't get updated as frequently as paid for software.
Have a look in OE and see if the email addy you mention is buried in any of the contacts or address books.