Microsoft pulled an early- morning prank straight out of the frat comedy "Animal House" on rival Netscape Communications yesterday.

But Mountain View-based Netscape quickly exacted its own "Revenge of the Nerds."

The high jinks started about 1 a.m., when a team of Microsoft engineers placed a car-size logo for Internet Explorer (a giant "e") on the front lawn of Netscape's headquarters.

Hours earlier at Fort Mason, Microsoft had released Explorer 4.0, the next version of its browser. A browser is software that finds and displays Web pages.

The Explorer team also left a greeting card with the picture of a grumpy baby with the note: "It's just not fair. Good people shouldn't have to feel bad. Best wishes, the IE team." A helium-filled balloon taped to the logo reads, "We love you."

About the only thing missing were strips of toilet paper on Netscape's front lawn.

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"It was all done in the name of fun," a Microsoft spokeswoman said.

Some officials at Netscape, which competes against Microsoft with its Navigator browser, weren't laughing.

"They have resorted to immature fraternity tactics to get attention," a Netscape spokeswoman said. "It's not something you'd expect from the largest software company in the world."

That didn't stop Netscape employees from retaliating.

Yesterday morning engineers placed Netscape's 6-foot-high dinosaur mascot, Mozilla, on top of the Explorer logo and spray-painted the "e" with the company motto: "Netscape Now."

They also erected a cardboard sign with the phrase, "Netscape 72, Microsoft 18," alluding to the company's shares of the browser market.

(In true tit-for-tat fashion, a Microsoft spokeswoman disputed the cardboard sign. She said the actual breakdown is 62 to 36 in favor of Netscape, according to Zona Research in Redwood City.)

The sophomoric practical jokes were on display most of yesterday.

Meanwhile, Netscape this week demonstrated new Navigator software that apes some of Explorer 4.0's key features. The demonstration at the Seybold Publishing conference in San Francisco was aimed at deflecting some marketing attention away from Microsoft's carefully choreographed Explorer announcement the night before.

Called Aurora, Netscape's new technology lets consumers view the Web pages, desktop files and e-mail from a single computer screen. It is similar to the Active Desktop feature of Explorer 4.0.

Aurora will come free with Netscape's Communicator browser, which is due in the first half of 1998 for $59. Communicator allows users to display Web pages while sharing documents and e-mail with others.