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.

Luckily, a test for the worm is easy, experts say. Try accessing a Web site for a security software site. Because the worm protects itself by preventing access to security sites, those infected won’t be able to see the Web sites.

And a download of Windows’ most recent security patch will remove the worm and protect the computer from any data breaches.

Computer worms, which differ from viruses in that they are independent programs (viruses need a host program), have caused headaches for home computer users and businesses in recent years as their creators seek to usurp secure data and wreak bandwidth havoc.

Already, the Conficker worm has infected a number of businesses and organizations worldwide, including millions of government computers in the United Kingdom.