Redmond has been marketing Windows 7 on the claim that it can boot up quicker than Vista.
However, Iolo Technologies, which sells PC tune-up software called System Mechanic, said that Windows 7 takes one minute and 34 seconds to become usable, whereas Vista takes only one minute and six seconds.
Iolo explained that most measurements were based on the time it took the desktop to appear. In Windows 7 this is much faster and can be about 40 seconds, but Windows 7 is not actually completely usable at that point.
The company decided to trip its stop watches on the time it takes Windows 7 to become fully usable "with CPU cycles no longer significantly high and a true idle state achieved".
Windows gets slower and slower as time goes on, Iolo added. The firm found that a three-month old machine can take up to a minute longer to boot, or two minutes and 34 seconds.
Windows 7 did outperform Vista at the three-month and six-month marks. But it "trailed the older version significantly" in its earlier boot-up tests.
Iolo plans to release more details on its findings and methodology next week.