It was always an unfriendly sort of rivalry, but lately the claws have come out and it has become even harder to imagine a happy ending.

Only a week after the launch of Microsoft’s new computer operating system Windows Vista, the company’s long-term rival Apple has claimed that the software is incompatible with its own music download service iTunes.

Vista was introduced with great fanfare last Tuesday. Bill Gates, the head of Microsoft, described it as the most expensive piece of software ever developed and said that it would usher in a new era of “connectivity” in home computing.

Apple is already claiming, however, that this connectivity does not extend smoothly to its own products: its online music store, iTunes, and its handheld music player, the iPod. It has advised iPod owners, who number at least 90 million, not to upgrade their computers to Windows Vista until the problems are sorted out.

Microsoft moved quickly to parry the blow and said that the company was working with partners including Apple to make sure that Vista was compatible with its software.

The battle between the two technology giants has lasted almost as long as the industry itself. Microsoft has dominated the PC market; its software now runs on 95 per cent of the world’s computers. Apple, with its Mac computer, built a smaller but fiercely loyal fanbase. Then in 2001 it successfully sidestepped into the new digital music market and is quickly becoming dominant.

Microsoft introduced its own music player, the Zune, only last November. “This is a place where we are deeply committed to being successful,” Mr Gates told the Consumer Electronics Show in Las Vegas last month. “You are going to see us in this space in a leadership position in the years to come.

Two days later in San Francisco, Steve Jobs, the flamboyant head of Apple, overshadowed the arrival of Vista by announcing the iPhone, a combined phone, music and video player and handheld computer.

Of the Zune he was scornful. It had a 2 per cent market share, he said, “So no matter how you try to spin this, what can you say.” Behind him a colossal video screen showed a Zune bursting into flames.

Then there were Apple’s television advertisements. In them, a besuited middle-aged man who looks not unlike Mr Gates represents a PC while a hip young actor plays a Mac.

Microsoft has attempted to rise above these gibes, but on Saturday Mr Gates complained to Newsweek: “Does honesty matter in these things, or if you are cool, that means you get to be a lying person whenever you feel like it?” It looked as though they were getting to him at last.

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