According to data from the firm, Windows 7, now accounts for approximately 14.6 percent of all operating systems in use, while Vista's share of the market has fallen to 12.6 percent.
Janco's numbers for April mark the first time that it has reported Windows 7's share as higher than Vista's.
But other operating system measurements clash with Janco's.
Net Applications, for example, pegged Windows 7's usage share during April at just 11.7 percent, under Vista's 15.6 percent.
Irish analytics company Stat Counter also said Windows 7 is still behind Vista, with the former controlling 13.5 percent of the OS usage market in April while the latter owned a 20.2 percent share.
Net Applications calculates its operating system share estimates by mining a pool of about 160 million monthly unique visitors to its clients' sites, while StatCounter claims that more than two millions sites use its free analytics tools.
Janco, on the other hand, uses data from just a handful of sites to come up with its numbers.
But Janco may only be jumping the gun.
According to projections based on Net Applications' data - and assuming the pace of Windows 7's increase and Vista's decrease for the last three months hold - Windows 7 will pass Vista as Microsoft's second-most-popular operating system before the end of June.
Net Applications' data also shows that by December 2010, Windows 7 will account for 24 percent of all operating systems worldwide, while Vista will have dipped below 10 percent for the first time since April 2008.
By all counts, the nearly-nine-year-old Windows XP continues to rule the OS roost, with share estimates running between 56.2 percent (Stat Counter) and 63.4 percent (Net Applications).
Although Windows XP is now in what Microsoft calls "extended support", it will continue to receive security updates until April 2014, making it the longest-supported operating system Microsoft has ever produced.
Microsoft has been trumpeting Windows 7's rapid adoption tempo for months, and specifically called out the move to the new OS as a major reason for the first quarter's 35 percent increase in net income compared to the same period in 2009.
At the time, Microsoft claimed that 10 percent of the world's PCs were running Windows 7.
Not surprisingly, the company did not mention Vista, part of the strategy one analyst said was Microsoft's successful move to "put Vista behind it".
According to Net Applications, Windows 7's share jumped by nearly 1.5 percentage points in April, the largest one-month increase since January, when the operating system gained 1.9 points based on brisk sales of PCs at the close of 2009.
Stat Counter had Windows 7's April increase at approximately 1.6 points, while Janco pegged the new OS's gain for the month at a whopping 2.4 percentage points.
During April, 19.7 percent of the visitors to PC Advisor's sister website Computerworld ran Windows 7, while only 13.4 percent ran Vista.
But traffic on technology sites such as Computerworld often skews toward newer operating systems, as well as toward non-Windows OS' such as Mac OS X and Linux.
Computerworld's Linux share for April, for example, was 4.8 percent, several times higher than the global share of just 1.1 percent that Net Applications reported; the site's Mac share of 14.9 percent was also significantly higher than Net Applications' 5.3 percent for Mac OS X.