Further expanding beyond its roots in Internet search, Google Inc. (GOOG) plans to launch a long-rumored program Wednesday that provides both text instant messaging and computer-to-computer voice chat.

The new program, Google Talk, will compete against similar free services offered for several years by America Online Inc. (search) , Microsoft Corp. (MSFT) and Yahoo Inc (YHOO). All are vying to increase their presence on PCs to boost online ad revenue and name recognition.

The launch was due to come two days after Google unveiled another free program that aggregates information on a computer desktop. It also comes less than a week after the company announced plans to raise $4 billion in a secondary stock offering — which some analysts speculated could be used to fund far-flung projects such as Internet telephony.

As a newcomer to messaging, Google could face an uphill battle.

AOL's messaging program has about 41.6 million U.S. users, followed by Yahoo Messenger with 19.1 million and MSN Messenger with 14.1 million, according to ComScore Media Metrix's (search) July report.

Users of those services are unlikely to switch unless the friends and colleagues on their "buddy lists" do the same. The top instant messaging services still do not communicate with each other, though promises of such "interoperability" have been made for years.

Google based its software on open standards, so it will work with smaller networks that are based on the same technology. Text messages can be exchanged with users of Apple Computer Inc.'s (AAPL) iChat, Cerulean Studios' Trillian and the open-source Gaim program.

Google also is inviting programmers to build its technology into their software.

"It means other people and developers will be able to add value to our network by being able to add this to computer games, productivity applications and anywhere else they want," said Georges Harik, director of product management at Google.

The new Google program features a basic user interface with few graphics, much like the main Google search site. It does not spawn pop-up windows or display ads like America Online's Instant Messenger.

"We'll have an uncluttered interface that allows you to search over your contacts pretty easily," Harik said. "It just stays out of your way unless you want to connect to someone."

Google Talk, which is being released in a beta test version, works only on PCs running Windows 2000 and Windows XP. Eventually, the company plans to release a version for Apple's Mac OS X.

Google Talk also requires users to have an account with the company's free Gmail e-mail system. Gmail previously was available only to those invited by a current account holder, but now Google is opening up registration to anyone in the United States.

Voice chat requires that both the caller and recipient have speakers and a microphone hooked up to their computers. It does not currently offer an adapter to which regular phones can be connected.

And unlike Internet phone services such as Vonage and Skype, Google's voice service does not support calls to the regular telephone system.

Harik also made clear that Google has no intention of trying to become a popular bridge to the other major instant-messaging providers. "We're not going to do anything like force other networks to interoperate with us," he said. "We're not going to arbitrarily break into their protocols."

However, since Google Talk runs on open standards, outside developers who incorporate the service into their programs could try to enable such interoperability.

Because of Google's large and loyal user base, the company's foray into instant messaging could threaten the other players, said Sara Radicati, head of The Radicati Group Inc., a technology research firm. As evidence, Radicati cited Google's entry into e-mail, when it became chic to have a Gmail account.

"We've seen people show off their Google address," she said. "It's on the level of `Hey, look at my new Swatch. I've got the yellow one while you're still wearing the blue.' ... It's a little thing, but it helps."