PC virus,spyware, error etc..

In one of my email addresses the management does not use spam, all those who use mail.com are just supposed to report it. What happened was someone is using my email address in the "from" and "subject" field and I saved every one of the emails (I don't know why, just to show someone if they can figure it out, I guess). There is about 5,000 emails in my InBox mostly from Hoodia Life and HGH Life (in the bottom of the content of the email) that is being bounced back to me from various mailer-daemon postmasters telling me basically that the email I sent (which I didn't) does not exist. You must know how it goes, "Sorry it didn't work out, I tried to send your email, etc." Anyway I've asked about this to others and thought since you're computer savvy maybe you could give me some additional info or let me know what to do about it. If you can, that would be really appreciated. If not, it's ok. I may just terminate the email address and forget about it but should it ever happen again I'd like to know what to do to stop whoever is doing it from using my email address. I don't understand how they can do it since only I used that email address and how can they obtain my personal computer number but if you ever heard of this kind of problem it would be good to know. Thank you should you be able to help. By the way, welcome to MyLot and good luck here.