As with Miakis, this happened to me when I fist got SL. Transformed my avatar completely. Luckily, I was a few days old so I did not yet buy an avatar or had a lot of inventory. I had to end up cancelling my account and creating a new one. I have learned since never to accept any attachments, and whoever gave you this is certainly not a friend. I would suggest filing a complaint to prevent this from happening to anyone else.