More than likely people you know have had their systems infected with the Klez worm, Gibe worm, Bugbear, or a similar 'virus'.

These randomly pick names from the system's address book, sending mail to one address as if it came from another, using random text from the infected machine. As the mail contains the virus, it will be returned to the apparent 'sender' by some systems.

There is also a possibility your system has the virus...

Symantec - Klez info and fix

Symantec - Gibe info and fix

Symantec - Bugbear info and fix