Grahame, it seems that you have one of the W32 trojans, I think they call them. they are received by you, quite innocently, and they go into your address book and start sending themselves to everyone in your book.
You had best get to McAfee.com or Norton and get an online scan-generally they are free for the first month.
don't sweat this,m it is happening everyday.. Just gety your system updated for about 29.95$$ per year...
it is well worth it.
HB