Critics contend that version 5.0 of America Online's Internet software — which a national technology magazine this week suggested was "the upgrade of death" — sometimes cripples existing Internet accounts with rival companies and prevents current AOL users from signing for service with competitors.

"You're faced with a company that knew its software would blow up the ability of its competitors," charged Bill Kirkner, chief technology officer for Prodigy Communications Corp., an AOL competitor that has roughly 2.2 million subscribers. "We can get our customers through it if they call, but the solutions are sometimes a bit nasty to go through." These include deleting and reinstalling software, and sometimes tinkering with arcane technical settings.

America Online, with 20 million subscribers, said complaints about interference by its latest software are overblown and the result of customers not understanding that if they click yes during installation to allow AOL to become their default Internet browser, AOL largely takes over all the online functions on the computer.

"If a member picks yes, we make their lives simple," said Jeff Kimball, AOL's executive director for its client software. That means AOL seizes responsibility to display all Web pages, send all e-mail and exclusively perform other tasks online.

But rivals and some AOL customers complain that the selection, made with a single click of a mouse with no added explanation, also can suddenly interfere with connections to rival Internet services or business accounts.

"It wipes out their previous settings, and the customer becomes an AOL customer," said Kirsten Witt, a spokeswoman for Mindspring Enterprises Inc., with 1.3 million subscribers. "In effect it allows the customer only to access AOL."

Peg Graham of New York installed AOL's latest software on her laptop weeks after its initial release in October with disastrous results: Her computer crashed. In vain, her laptop manufacturer urged her to reinstall her entire Windows operating system — she did three times — before she finally paid a local repair shop $145 to fix it.

Afterward, she returned to an earlier version of AOL's software she considers less risky. She suspects the new program suffered conflicts with the laptop's network hardware she used to connect at her university.

"There's no person to hold accountable," fumed Graham, who's now shopping for a new Internet service. "They just say, yes, we know there might be problems. It's almost like brushing you off."

The complexity of modern software can lend itself to problems that are hard to diagnose and make it even harder to lay blame. Rival Internet providers won't say exactly how many customers have reported problems, and no one admits even to calling AOL formally to complain about its software's alleged behavior.

AOL spokeswoman Anne Bentley reported "very minimal calls about this," and many AOL customers said they installed the latest software without hassle.

But AOL's own message boards, with thousands of complaints since Christmas, suggest these problems are more than fantasy concocted by disgruntled rivals. And this week, Windows Magazine's Web site asked, "AOL 5.0: The Upgrade of Death?"

The magazine's technical testing showed AOL's software installed redundant files that threatened a computer's stability. The software crashed the first time it ran. "AOL can reduce a perfectly good computer system to a paperweight," the magazine concluded.

Software problems like these also can take on enormous implications when a company becomes as dominant as AOL, which last week announced its $145 billion mega-merger with Time Warner Inc. That's a deal that will allow AOL to distribute this new software with Time Warner products, including its magazines, which draw 120 million readers. So far, about 8 million of AOL's 20 million customers have installed the new software.

The federal government last year pursued high-profile antitrust complaints against Microsoft and Intel, proving that even goliaths of the nation's booming high-tech industry aren't beyond its regulatory scope.

Indeed, as AOL Chairman Steve Case testified before Congress last April about high-speed Internet connections, he drew a mild rebuke from Sen. Ernest Hollings, D-S.C., who cautioned that AOL one day could run up against antitrust laws because of its own dominance.

"They're still a young-company mentality," said Frank Soler of San Francisco, an AOL subscriber since its earliest days who won't install the latest software for fear of its effects. "They could find themselves in a heck of a lot of trouble. Somebody might accuse them of trying to do away with competitors. They have to be very careful how they proceed."