(2nd part - Sent after I've just spotted all of the spelling & grammatical errors in the first part!):
3. Simple, straightforward spam. (i.e. the spammer has just put 'returned mail' in the title line instead of 'Important Account Information" or whatever) If. when you've opened those mails, you've found ads for fake watches, etc then this will be the cause.
4. Someone has hijacked your computer! This shouldn't happen if your firewall is working OK but it's just a possibility. Some spammmers (and the vast majority of virus creators) are very careful to ensure that mail can't be tracked back to their computer. Instead, they route everything through someone else's computer so that it appears to come from someone who is totally innocent. While this could be the cause, it's highly unlikely - it's usually the computers of big companies which are at risk. If this is happening you'll know because you'll have constant internet traffic on your computer (which would probably slow your browser to a crawl).
So, you've probably got nothing to worry about but, just to be on the safe side, check that your anti-virus and firewall programs are fully up to date (and properly configured). Then run a full virus scan. In future, just ignore anything similar. (We nearly all get these types of mail, sooner or later!).
Chris