As if Steve Ballmer didn’t have enough to worry about after last week’s Sidekick/Microsoft (MSFT) Danger debacle, here’s a bar graph that may add to his miseries.

The graphic (shown full-size below the fold) comes out of a report to clients issued Monday by Broadpoint AmTech analyst Brian Marshall. Anticipating the release of Windows 7 next week (Oct. 22), Marshall reviewed Mac sales figures over the past 10 years to analyze the impact of the four previous Windows launches:

• Windows 98 (launched on June 25, 1998)
• Windows 2000 (launched on February 17, 2000)
• Windows XP (launched on October 25, 2001)
• Windows Vista (launched on January 30, 2007)

His findings:

“We have concluded that no negative correlation exists on Apple’s (AAPL) hardware sales when Microsoft launches a new OS. Ironically, we believe new OS launches from MSFT may have even acted as a ‘delayed accelerant‘ to AAPL’s computing sales.”

The chart suggests that Vista’s well-publicized problems were the real accelerant, providing fodder for more than two years of Get A Mac ads.

[Click to enlarge]

Correlation is not causation, of course, and Marshall is quick to add that “AAPL’s success (or failure) in the computing market is largely idiosyncratic (or company-specific) in nature and not dependent on others in the industry.”

Still, he thinks that despite Windows 7 — if not because of it — Apple could double its global PC market share from about 4% today to 8% over the next five years.

[Follow Philip Elmer-DeWitt on Twitter @philiped]