Service allows registered users to send instant messages or talk over the web to other customers.

Google yesterday fired a second broadside this week in the war against Yahoo, Microsoft and AOL with the launch of Google Talk, a service that allows registered users to send instant messages or talk over the web to other users.

The release comes two days after the unveiling of Google Desktop, which includes a new Sidebar and an upgraded version of the current Desktop Search.

Google Talk hopes to transform the instant messaging market since it uses open standards in a market where the main rivals prevent users from talking to one another.

Google's director of product management, Georges Harik, says that, by opening up the interface to third parties, "we hope to make a fairly big change to the way messaging is done".

Instant-messaging software from AOL, Microsoft, Yahoo and Skype is free, but is seen as an important money-making tool because it increases traffic to those sites, which in turn helps generate more advertising or subscription revenue.

Also, as companies offer more news and entertainment on their sites, having a communications tool to deliver and distribute that content is increasingly important.

Google comes relatively late to the game, almost a decade after AOL launched the first version of its Instant Messenger service. Users typically sign up in groups, creating so-called buddy lists of co-workers, friends and family they communicate with.

Skype, which has attracted 51 million users worldwide over the past two years, announced plans yesterday to allow its service to operate with other applications or websites. And MSN launched a new version of its instant messaging service on Tuesday that will link to Vodafone Messenger, which is offered by Vodafone.

Both Google's launches will arouse industry interest that could stimulate Google's share price, which has slipped recently following the announcement that the company was raising a $4 billion (€3.25 billion) war chest by selling more shares.

Google's entry into the instant messaging market was predictable. The company has spent the past 18 months converting itself from a search engine into a portal that can compete head to head with Yahoo, Microsoft and AOL.

The problem with current messaging systems is that, in general, users can only send messages to other people on the same network. Google wants to change that, opening its interface and supporting a standard protocol. This means some existing messaging programs, such as Trillian and Jabber, will already work with Google.

What is also significant is that this is Google Talk and not Google Messenger or even Google Chat. Other messaging systems have added voice features over the years, but it's likely to be a major element in Google's service.

This could also hit pioneering internet telephony firm Skype, which has been hugely successful in persuading users to download proprietary software to do something their PCs could do already - though arguably not with the same quality or convenience.

Google's Voice over Internet Protocol (VoIP) service will be standards-based, and the company has licensed the same third-party code as Skype.

The main drawback with the internet telephony service is that, at the moment, it can only be done with Google's client software, which is only available for Microsoft Windows. Also, Google Talk will only be available to Gmail users, and will use the same contacts list.