Spammers use "real" addys in the "from" line because their emails stand a better chance of being opened, which either leads to a sale or confirms that the email addy is valid. (If you open a spam that has graphics (pics) hosted from another source, when the email is opened the pics are retrieved, thereby letting the spammer know you peeked.)
You can also beg your friends to delete headers before forwarding, and use the BCC line. Yes, mutual friends will send you the same emails twice, but spammers harvest email addys from all the clutter people don't bother to delete. Companies like Yahoo & Incredimail are even turning their junk into graphics that are harder and harder for the novice to clean out of an email they want to forward. Sometimes it is faster to cut-n-paste the message of the email and send a new one.
You can tell your friends you are not a spammer, and that this will pass. Meanwhile, sign up for a free email acct. somewhere and use it for anything "questionable". We call them "throwaway" addresses. GMail has some of the best spam filtering I've ever used, btw.