Restating some general principles:

It's unlikely you have a "virus/worm that sends e-mail to friends."

If someone gets your contact list, they can send emails to everyone you know - pretending that they com from you; but they don't have to be sent from your computer.

The real danger for your computer is that there's might be a worm/virus on your computer that allows someone to look at (and usually to record elsewhere) everything on your computer and everything your computer does. If someone is sending emails with your name on them to people on your contact list, it is possible that they've placed a "spyware" infection on your computer, but that's not the only place they can get your contacts.

Most email services keep your contacts and your email on their servers. Someone can hack into the server that has all your information and get everything that's there, about you and about everybody else using that service, Because there's lots of information on the servers, and probably pitifully little of real interest on your computer, it has become much more "popular" to attack the servers than to try to suck up the thousands of individual users needed to get the same information.

Since attacks on individual users and on mass servers are both still fairly common, you do need to maintain good malware protection on your own computer, keep it updated, schedule it to run frequently, and if your program can, set it for "real time scans" of all incoming stuff.

It's fair to expect servers to have better protections than you can get, and most of them do; but it would be difficult to name any major service/server that has not experienced successful penetrations and loss of data. (When Sony says "nothing valuable was taken" they mean nothing valuable to them. Your stuff that was there is apparently "not very valuable.")

Unfortunately, once "they" have your contact list, and possibly some or all of your passwords, etc., you can't erase their copies or get them back. You probably will want to change all your passwords and may want to switch to a different email account. If you change email, people getting the strange stuff should be able to block any new incoming from your old one, but that depends on:

1. They know they need to do it.
2. They're interested/concerned enough to do it.
3. They can figure out how to do it on their own computers.
4. They get it done.

Don't count on any of the above, although you can "encourage" significant contacts.

At a second level of attack, you could have malware on your computer that allows someone else to "take over" the computer and to do anything on it that you can do. This probably would be necessary for someone to send emails to people in your contact list from your computer. Machines infected with such "bots" are usually assembled into groups to form "botnets" that can be activated (by a single command sent simultaneously to all the "bots") to follow preloaded instructions for mass DDoS attacks on individual servers. A particular danger is that many fairly large (>100,000 computers) are controlled by "botmasters" too young to have developed "moral values." Steve Gibson identified one 13 year old "botmaster" running about 100,000 "slave bot" machines somewhere around 30 years ago, IIRC.

It doesn't really matter what's on your computer, since the "botmasters" are only interested in whether they can control it to attack someone else, with a minor concern about whether your computer and connection are "fast enough" to be worth adding to their "net."

Removal, or even detection, of botware can be quite diffult. Some can be detected by AV programs, but detection is often only by observing changes in your computer's behavior. If there's a real reason to suspect such an infection you probably need expert help, or you can reformat and start from scratch.

(Having just done a start-over for other reasons, I can report that it went quickly. Only about 37 hours to get almost everything back onto the replacement C:\ drive. No matter how good your backups are, the stuff has to be copied at a rate your machinery can handle.)

John