public static class VersionHelpers
{
    public static bool IsWindows7OrGreater()
    {
        var winver = new RtlOsVersionInfo();
        winver.OsVersionInfoSize = Marshal.SizeOf(winver);
        RtlGetVersion(ref winver);

        return
            (winver.MajorVersion > 6 ||
            (winver.MajorVersion == 6 & winver.MinorVersion >= 1));
    }

    [DllImport("ntdll.dll")]
    private static extern int RtlGetVersion(ref RtlOsVersionInfo versionInformation);

    [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Unicode)]
    private struct RtlOsVersionInfo
    {
        public int OsVersionInfoSize;
        public int MajorVersion;
        public int MinorVersion;
        public int BuildNumber;
        public int PlatformId;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)]
        public string CsdVersion;
    }
}