Microsoft's update service is more reliable than those offered by and Linux-distributor Ubuntu, a Web monitoring firm said Friday.

In the second quarter of this year, Update was up 100% of the time, Pingdom, which monitors uptime and performance of Web sites and servers, Apple's Software Update had a "respectable" 99.9% uptime, and Ubuntu fared much worse at 98.64%.

In terms of total downtime for the three-month period, Apple's service was unavailable two hours and 34 minutes and Ubuntu one day, five hours, and 45 minutes, Pingdom said. While Ubuntu finished last, Pingdom noted that the organization's repositories have mirrors around the world, so users can packages from those as well.

Microsoft and Apple have automated services to send updates to users of Windows and Mac X, respectively. Ubuntu distributes updates through its main repositories. Pingdom based its findings on the uptime of the services' access points, which are where computers on the Internet connect to the services.

Apple download servers took a beating Friday with the release of the iPhone 3G. With people worldwide buying and activating the media player-smartphone hybrid, The same experience was reported by people trying to update their older iPhones with a new operating system, also released Friday.

Microsoft this week for its Windows XP operating system. Though not a major update, Windows XP SP3 offers numerous enhancements over the SP2 version of the OS. It includes all updates issued since Windows XP Service Pack 2 was released in 2004, as well as some new elements.