The infamous MyDoom worm has morphed again into MyDoom.F, a denial of service attacker which is targeting Microsoft as its predecessors did, but is also going after the Recording Institute Association of America (RIAA).
Users get infected through the usual means, by opening attachments on insecure emails, allowing the worm to spread itself via the Windows System directory.
Random email messages are generated, including "You use illegal file-sharing ... your IP was logged", "Approved", "Attention", "Your credit card" and "Expired account." Fears are that such email attacks are becoming more personal and harder to spot. Source