Paracab,
The way most viruses propagate is by going to the infected computer and collecting all the addresses in the infected computer's address book. Then they go though all the emails ever sent or received on the infected computer and collect all those email addresses.
Then the viruses, send an email to each of those collected email addresses along with a copy of the virus attached to the emails.
So do NOT trust anything you get from a friend. Be suspicious of everything!
Take Care,
Richard
P.S. You still have not told us the extension of the file you cannot open.