I have a virus

[MistyChelle]

First I want to say that I'm very new to Avast, so if I sound a little vague that's why. I also don't know a great deal about computers.
I installed Avast last week after a friend recommended it over the old AVG I had.  It does seem to be working but I'm still learning about it.
I got an email from a friend today saying that his daughter recieved an infected email from me. I did not send it and I don't even know how her address got in my hotmail contacts folder, I didn't put it there.  
I ran the avast on a thorough scan when I installed it and I ran it again today (both times took 3 hrs to scan my system). Neither time picked up any virus on my system and I'm at a loss as to why. Any ideas?

[igor]

Sure.
Most of today's viruses/worms fake the sender address. So, if somebody receives an infected e-mail having your address in the "From" field, it doesn't really mean that it was sent by you. Anybody having your e-mail address (e.g. anybody who ever received an e-mail from you) could be the sender of the e-mail (if he/she got infected).

[MistyChelle]

Thank you for your reply  
I forgot to mention in my post that both my husband and I (sep addys) have had "return emails" stating that an email could not be sent because it contained a virus and that it should be cleaned before resending.
Also, when I recieve an email from the friend who introduced me to Avast it has that the email has been checked by avast at the bottom, mine do not have this. How do I get Avast to check my outgoing mail this way?
Thanks again.

[Lisandro]

Also, when I recieve an email from the friend who introduced me to Avast it has that the email has been checked by avast at the bottom, mine do not have this. How do I get Avast to check my outgoing mail this way?

1) Left click the 'a' blue icon in your system tray.
2) Choose Internet Mail provider (for Outlook Express) at left or Outlook/Exchange provider (for MS Outlook)
3) Click on Customize
4) Check the options for Inbound Mail and/or Outbound Mail: scan and add a note into 'clean' messages

Could you do this?
Wellcome to forums.