clueless
This has nothing to do with whether a release is major or minor. (All 4 releases that you mention are "major" - since Microsoft asked for money to upgrade. Service packs are free, and are therefore minor releases.)
The internal version number (5.* and 6.*) are mostly invisible to the user, and are available for privileged software to query the OS to determine the API support. If your driver uses V5.0 APIs, then V5.* is fine. If you use an extension released in V5.2, you'll check for that.
Although I can't imagine finding anyone who doesn't consider Windows 7 to be a major update from Windows Vista, at the privileged API level it was a minor, mostly compatible, update. Therefore, Vista and Windows 7 are both major version 6.
The minor version was updated from .0 to .1 so that software that depended on extensions in Windows 7 could make intelligent decisions. (For example, the installer might check major/minor versions to choose which files to install - or whether to generate an error message and abort.)
Windows 2000 (Version 5.0)
Windows XP (Version 5.1)
Windows Vista (Version 6.0)
Windows 7 (Version 6.1).
So the only reason XP and 7 are considered major version is the name? According to Microsoft they are only .1 releases.
This has nothing to do with whether a release is major or minor. (All 4 releases that you mention are "major" - since Microsoft asked for money to upgrade. Service packs are free, and are therefore minor releases.)
The internal version number (5.* and 6.*) are mostly invisible to the user, and are available for privileged software to query the OS to determine the API support. If your driver uses V5.0 APIs, then V5.* is fine. If you use an extension released in V5.2, you'll check for that.
Although I can't imagine finding anyone who doesn't consider Windows 7 to be a major update from Windows Vista, at the privileged API level it was a minor, mostly compatible, update. Therefore, Vista and Windows 7 are both major version 6.
The minor version was updated from .0 to .1 so that software that depended on extensions in Windows 7 could make intelligent decisions. (For example, the installer might check major/minor versions to choose which files to install - or whether to generate an error message and abort.)