I am running Os X Lion with Home and student office for Mac. My MS Outlook Mail programme has picked up a virus I think. The programme is sending out multiple mails to addresses that I have no knowledge of.

It is called "spoofing" and the problem isn't with your Mac.

What happens is that a person with a pc is attacked by malware that uses that person's address book to send emails looking like they come from all the contacts in his list,

Unfortunately, there's no way you can stop it because your Mac isn't involved. You can alert your PC friends but that's about the extent of what you can do. It should pass in time.

Or, you may want to change your email address, temporarlily.

Only if you make sure every PC friend with you in their address book is always running the latest version AV software for PC. So, basically... nothing.

Although I concur with macjack's answer, changing your email address can be very inconvenient for some people, since they may have been using it for years. If you can't change your address, at a minimum you should change your email password so you know that you are maintaining control over your email account.

Paul111 wrote:

A couple of days ago I noticed 14 emails in my inbox and others in Spam and Junk with a header explaining that these mails are undeliverable; they are mail addressees that I have never heard of.

Check your "Sent Mail" folder to see if any of those messages were sent from it. If not then they just spoofed your address. If any are found there then your account was hacked (almost certainly on your ISP's mail server) and you need to change password and/or e-mail address.