Spammers use your eMail return address to keep themselves from getting blocked. There is nothing you can do about this, unless someone figures out a way to handle the problem network-wide.
Bob here – This is also a technique used to spread viruses and spam in general. The fact that your email address gets used means that the infected machine belongs to <i>someone</i> who has emailed you at some time – and has your address in their list.
See this similar question and answers.
This is not necessarily a virus infection. It is called mail “Spoofing”. The message appears to be sent by a user. The Spammers use a combination of characters and sends messages out “as a User”. Unless there is an SMTP authentication security in place you cannot do anything about it.