A particularly nasty computer worm that's already infected up to 15 million machines worldwide is set for another attack on April 1. The so-called Conficker worm, which burrows itself onto computers running Microsoft's Window's operating system, has been spreading since at least October, according to Microsoft. Since then, its creators have tweaked the nefarious software to stay ahead of security fixes. The worm, which relies on Internet connections to receive instructions from Web sites online, has spread through e-mails and downloads over the past few months. Microsoft and computer security software developers like McAfee and Norton already have fixes available for users.