Virus Attacking My Account

You need to make sure exactly what is going on here.  Look at the emails you are getting back.  View the return headers.  You can select to view full headers from the view menu of most email programs. 

Who are the emails you are receiving addressed to?  is it you?  More importantly who are they coming from? 

Odds are it's from email servers telling you that your email did not go through for whatever reason.  If this is the case YOU DO NOT HAVE A VIRUS.

What has happened is that a spammer has collected your email address and is using it as a fake return address on bulk emails they are sending out, probably trying to sell people Viagra.

Hackers use robot programs to collect email addresses from around the Internet and sell them to be used by spammers.  It's trivial to fake the senders email address and most email servers will not accept email if the sender is not valid.  That's why they "borrow" your address.

There is nothing you can do to stop this other than being vigilent and don't list your email address anywhere.

There is the chance that your PC has a virus but your AV software should take care of that.

Also some emails have malicious payloads and when opened they run scripts that forward themselves to your address book.  Again, a good anti virus tool will fix this.  Avast has an email scanner can stop this.