NEW YORK — What began as a ninth-grade prank, a way to trick already-suspicious friends who had fallen for his earlier practical jokes, has earned Rich Skrenta notoriety as the first person ever to let loose a personal computer virus.

Although over the next 25 years, Skrenta started the online news business Topix, helped launch a collaborative Web directory now owned by Time Warner Inc.'s Netscape and wrote countless other computer programs, he is still remembered most for unleashing the "Elk Cloner" virus on the world.

"It was some dumb little practical joke," Skrenta, now 40, said in an interview. "I guess if you had to pick between being known for this and not being known for anything, I'd rather be known for this. But it's an odd placeholder for (all that) I've done."

"Elk Cloner" — self-replicating like all other viruses — bears little resemblance to the malicious programs of today. Yet in retrospect, it was a harbinger of all the security headaches that would only grow as more people got computers — and connected them with one another over the Internet.

Skrenta's friends were already distrusting him because, in swapping computer games and other software as part of piracy circles common at the time, Skrenta often altered the floppy disks he gave out to launch taunting on-screen messages. Many friends simply started refusing disks from him.

So during a winter break from Mt. Lebanon Senior High School near Pittsburgh, Skrenta hacked away on his Apple II computer — the dominant personal computer back then — and figured out how to get the code to launch those messages onto disks automatically.

He developed what is now known as a "boot sector" virus. When it boots, or starts up, an infected disk places a copy of the virus in the computer's memory. Whenever someone inserts a clean disk into the machine and types the command "catalog" for a list of files, a copy gets written onto that disk as well. The newly infected disk is passed on to other people, other machines and other locations.

The prank, though annoying to victims, is relatively harmless compared with the viruses of today. Every 50th time someone booted an infected disk, a poem he wrote would appear, saying in part, "It will get on all your disks; it will infiltrate your chips."

Skrenta started circulating the virus in early 1982 among friends at his school and at a local computer club. Years later, he would continue to hear stories of other victims, including a sailor during the first Gulf War nearly a decade later (why that sailor was still using an Apple II, Skrenta does not know).

These days, there are hundreds of thousands of viruses — perhaps more than a million depending on how one counts slight variations.

Although some of the early viruses overwhelmed networks, later ones corrupted documents or had other destructive properties.

Compared with the early threats, "the underlying technology is very similar (but) the things viruses can do once they get hold of the computer has changed dramatically," said Richard Ford, a computer science professor at the Florida Institute of Technology.

Later viruses spread through instant-messaging and file-sharing software, while others circulated faster than ever by exploiting flaws in Windows networking functions.

More recently, viruses have been created to steal personal data such as passwords or to create relay stations for making junk e-mail more difficult to trace.