Microsoft last week patched nine vulnerabilities, five marked "critical," in Windows 7, a move that will require users upgrading to the new operating system starting Thursday to download a security update to keep their PCs secure.

The patches were the first for Windows 7's final build, dubbed RTM for "release to manufacturing," that has been in some customers' hands, primarily enterprises with volume licensing agreements, since August.

Windows 7's patch count, however, was significantly less than either Windows Vista's, its immediate predecessor, or that of Windows XP, the eight-year-old operating system installed on the majority of systems worldwide.

An analysis by Computerworld of the massive 13 October security update -- the largest by Microsoft since it started patching on a regular monthly schedule six year ago -- showed that Windows 7 was affected by nine of the 34 vulnerabilities, or 26% of the total. Its count of critical bugs -- the most serious as marked by Microsoft -- was five out of a possible 21, or 24%.

Windows Vista, meanwhile, was impacted by 19 of the 34 vulnerabilities, or 56% of the total, with 11 pegged critical, or 52% of the possible.

Windows XP was affected by the most vulnerabilities of all: 24 out of 34, or 71% of the total. Of the two-dozen bugs that needed patching in Windows XP, 18 were tagged as critical, or 86% of the total critical count.

The tallies indicated that, at least this month, Windows 7 was afflicted with about half as many vulnerabilities as was Vista, and about a third as many as was Windows XP. Its critical bug count followed the same pattern.

Those flaws that did affect Windows 7 were probably due to recycled code, said security experts. "The Windows 7 vulnerabilities are coming from its legacy code base," said Jason Miller, the security and data team manager for patch management vendor Shavlik Technologies.

In fact, none of the vulnerabilities patched last week were Windows 7-only flaws. Of the five security bulletins that affected the new OS, all five also impacted Vista and XP, and all but one had to be patched in the even-older Windows 2000.

The phenomenon of vulnerabilities in old code isn't new. Just months after Vista's launch, security researchers took Microsoft to task for overlooking a vulnerability in Windows' animated cursor in the brand-new operating system, even though was closely linked to a bug Microsoft had patched more than two years before.

Microsoft has promoted Windows 7 as safer and more secure than its predecessor, something it did two three years ago when it touted Vista as better than XP. But everything is relative.