Someone has obtained my contact list of names. Then I get emails from this person, which says it is forwarded from a person on my contact list, and the email promotes a product or website. I know that the email is actually not from a person on my contact list. How do I prevent this from happening, or correct it??
Check your Sent folder to see if anything is in there that you know you didn't send. If there is, then the account has been compromised. In that case you need to change the password to something a lot stronger than whatever it is right now, and also change the secret question and answer for the account.
If the Sent folder looks OK, then the most likely thing is that your account is being "spoofed". Spammers can make it look like emails came from you when in fact they did not. If that is the case, there is nothing you can do except notify your contacts of the problem and wait it out. Once a spammer has your email address there is nothing you can do to stop it. But in order to avoid detection, they will usually only use an address for a few days or a week and move on to annoy someone else.
Also, use your A/V software to run a complete scan of your machine to see if it can find anything wrong.