NOTA - there are numerous threads on this forum about forged email origins and addresses. Have a look back (search?).
In a nutshell, the 'nasty people' put a virus into someone's machine, which trawls for ALL the email addresses on that machine and then sends to and 'from' each of them trying to lodge its sad package there.
If it is not coming from your's for sure, then one of your acquaintances with your address in his address book has it or it has found your address somewhere on the web.
DO HEED BEAGLE! Do not open any strange attachments.