Google's claims that Microsoft's built-in Vista desktop indexing and search tool violates its antitrust agreement could be difficult to prove even if the software does slow down the performance of Google's competitive Google Desktop offering.

As long as a user can run alternative software to Microsoft's Instant Search software, it's unlikely that US federal antitrust officials would consider coming down on the software giant, analysts and users said.

Google's claims are far different than the ones posed by Netscape during the browser wars that led to the Department of Justice's antitrust suit in the 1990s, said Rob Helm, research director at Directions on Microsoft in Kirkland, Washington. "Microsoft beat Netscape in part by leveraging its relationship with PC manufacturers," he said. "This is a lot subtler."

Google seems to be alleging that "if two pieces of software don't play together, then it must be an anticompetitive tactic of Microsoft's," Helm said. "I don't recall any past antitrust cases asserting something so broad," he said.

The presence of Microsoft's Vista desktop search could be slowing down Google's product merely as an accident of product design, not because of any malicious intent by Microsoft, Helm said.

"Even if Microsoft's software was perfectly written, the way Google interacts with it might be bad, and either company might be at fault," he said. "They could have both done the right things in different ways that might conflict with each other."

According to a report in the Wall Street Journal, Google sent a white paper to US federal and state antitrust officials in April to try to convince them that Vista makes it difficult for consumers to use rival desktop search software.

In its white paper, Google claims that Vista's search boxes and bars - available in several places in the OS, including the Start menu and in the Windows Explorer file manager - work only with Microsoft's search and indexing tool. The company also said it is nearly impossible to turn off Vista's indexing, which means a competitor must add a second indexer that slows down a PC.

Google spokesman Ricardo Reyes confirmed the company's charge against Microsoft on Monday.

Microsoft is disputing Google's charges and said that it has worked closely with federal officials to ensure its Vista OS, released to consumers in January, fosters rather than inhibits competition in the area of desktop search.

Users can disable Vista's desktop search service, but the company has not made it simple for them to do so, acknowledged Jack Evans, a Microsoft spokesman. He said this is because the company designed Vista's desktop search specifically "to not affect performance and back off any other programs running" - including any third-party desktop search software - in a way that should resolve any claims of anticompetitive behaviour, Evans said.