Apple has posted a fix on its Web site for a serious problem that causes its Macintosh computers to seize up when users attempt to upgrade to the company's new Leopard operating system.

"It may be necessary to perform an Archive and Install installation of Leopard," Apple says in the support bulletin, which appeared over the weekend.

The workaround moves existing files on a user's computer to a folder named Previous System, over which the new software is installed. "Applications, plug-ins, and other software may have to be reinstalled after an 'Archive and Install,'" Apple warns.

Apple launched Leopard -- officially known as OS X 10.5 -- on Friday amid considerable hype. The operating system offers numerous graphical and security enhancements designed to entice computer users away from Microsoft's Windows Vista. Within hours of the launch, however, users were reporting the so-called 'Blue Screen of Death' problem on the company's online support forums.

"I was so excited to get my hands on this software and momentously disappointed with the results," said a post from 'Christian Jones1' of North Wales, U.K. Jones said his Mac froze up after the Leopard installation process was only 5% complete.

Thousands of similar posts indicate the problem is widespread.

For its part, Apple says software on users' machines that may not be Leopard-compatible is to blame. "You may have third-party 'enhancement' software installed that does not work with Mac OS X 10.5 Leopard," the company said in its support post.

Some on the Apple forum said the problem was due to a glitch in a third-party program called APE (Application Enhancer), created by developer Unsanity. "It looks like Application Enhancer is NOT compatible with Leopard," wrote poster 'Jon Thornburg.'

Leopard's 'Blue Screen of Death' troubles could prove particularly embarrassing for Apple, which prides itself on offering a simpler, more user-friendly alternative to Microsoft's computing environment. Now, as Apple aims for a bigger chunk of the OS market, its customers are encountering a problem all too familiar to Windows users.