one of the common practises of virus writers is known as address spoofing.
as most virus are wrtitten with their own e-mail sender, it is easy enough to pick up all the addresses in a book and use them to send out infected email purporting to be from a legitimate person.
sometimes they use the mailer daemon trick advising that an emial you allegedly sent was undeliverable.
you could infect your computer by opening those as well.
as for the scan there are several sites where you can do a free on line scan to ensure you are clean
one such is
http://www.ravantivirus.com
please click on the link above and look on the left side of the browser window for a link that says 'scanonline'.
david williams