Microsoft's Windows beat operating system rivals Mac OS X and Ubuntu in a three-month test of update server uptime, a Swedish uptime monitoring company said.

According to Pingdom, Microsoft's update servers were available to users 100 per cent of the time during the three-month stretch of April, May and June, with no apparent down time. Apple's update servers, meanwhile, were offline for 2 hours and 34 minutes during that period (99.9 per cent uptime) while the main repository of Ubuntu, one of the most popular consumer distributions of Linux, was dark for 1 day, 5 hours and 45 minutes (98.6 per cent uptime).

In a post to its company blog last week, Pingdom outlined how it pinged the update servers and Ubuntu repository every five minutes during the second quarter. Any offline indication was confirmed from two different locations, said the company.

Pingdom monitored the Windows Update Web site URL www.update.microsoft.com, Apple's Software Update server at swscan.apple.com/content/catalogs/index-1.sucatalog , which contains the index necessary to receive new updates, and the main Ubuntu repository at archive.ubuntu.com .

Microsoft and Apple use their respective update services to provide security patches, other fixes and for larger-scale operating system upgrades. Ubuntu users, on the other hand, have a wide range of locations, including numerous mirrored sites, to download new versions of that Linux operating system, a fact not lost on its Pingdom.

"It should be noted, though, that Ubuntu's repositories have mirrors around the world, so users can download packages from those as well," the company said in the blog post.

Ubuntu supporters stressed that, and other factors, as they defended their operating system in comments appended to the Pingdom blog.

"You've tested one out of a multitude of Ubuntu's update servers," argued a user identified as "Oli" in one comment. "On installation, you pick a local mirror to speed things up. Now, this isn't as clean as a transparently geo-mirrored solution as Microsoft, but you're just plain wrong to suggest that if the main repo went down now, nobody could get updates."

"If we look at the dates, Ubuntu [8.10] came out on 24 April," said "evenorbi" in another comment. "Around release time usually the repositories are very busy because of the updates/installs, so of course they are down most of the time."

Some got their facts wrong. "Neither Apple nor Microsoft offer full operating system upgrades via their upgrade systems," said someone named only as "Jake" in a third comment. "They are just security updates and so forth, so there is much more load on Ubuntu servers."

That's not the case, however, as both Microsoft and Apple issue major operating system upgrades regularly via their update services. In Microsoft's case, it dubs these updates "service packs;" among the most recent was Windows XP Service Pack 3 (SP3), which began reaching users automatically last week.

Apple also delivers larger upgrades through its Software Update. Last month, for example, it issued Mac OS X 10.5.4, an 88MB update.