This is probably the Klez32 worm, and I've been getting my share of these too. As far as I understand it, a computer infected with this will, totally unbeknownst to the hapless owner, pick names and addresses out of their mail list and fire these things off at random. The return address and message is faked up, I think also with names from the mail list.

I have yet to identify a specific source for the many copies of this that I have gotten--there seems to be no pattern at all to them. But I will confirm that they do seem to come in "waves," that is, a bunch in a few days then none at all for a long time.

Keep them antivirus programs running and updated folks!