System.Environment.OSVersion has the information you need for distinguishing most Windows OS major releases, but not all. It consists of three components which map to the following Windows versions:
+------------------------------------------------------------------------------+
| | PlatformID | Major version | Minor version |
+------------------------------------------------------------------------------+
| Windows 95 | Win32Windows | 4 | 0 |
| Windows 98 | Win32Windows | 4 | 10 |
| Windows Me | Win32Windows | 4 | 90 |
| Windows NT 4.0 | Win32NT | 4 | 0 |
| Windows 2000 | Win32NT | 5 | 0 |
| Windows XP | Win32NT | 5 | 1 |
| Windows 2003 | Win32NT | 5 | 2 |
| Windows Vista | Win32NT | 6 | 0 |
| Windows 2008 | Win32NT | 6 | 0 |
| Windows 7 | Win32NT | 6 | 1 |
| Windows 2008 R2 | Win32NT | 6 | 1 |
| Windows 8 | Win32NT | 6 | 2 |
| Windows 8.1 | Win32NT | 6 | 3 |
+------------------------------------------------------------------------------+
| Windows 10 | Win32NT | 10 | 0 |
+------------------------------------------------------------------------------+
For a library that allows you to get a more complete view of the exact release of Windows that the current execution environment is running in, check out this library.
Important note: if your executable assembly manifest doesn't explicitly state that your exe assembly is compatible with Windows 8.1 and Windows 10.0, System.Environment.OSVersion will return Windows 8 version, which is 6.2, instead of 6.3 and 10.0! Source: here.
Update: In .NET 5.0 and later, System.Environment.OSVersion always returns the actual OS version. For more information, see Environment.OSVersion returns the correct operating system version.
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