C# Definitions:

// Use this when the unmanaged API expects the structure passed by-value, or
// or if you want to pass it by-reference as a pointer to a structure
struct OSVERSIONINFO {
    public uint dwOSVersionInfoSize;
    public uint dwMajorVersion;
    public uint dwMinorVersion;
    public uint dwBuildNumber;
    public uint dwPlatformId;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
    public string szCSDVersion;
    public Int16 wServicePackMajor;
    public Int16 wServicePackMinor;
    public Int16 wSuiteMask;
    public Byte wProductType;
    public Byte wReserved;
}

// Use this when you want to pass it by-value even though the unmanaged API
// expects a pointer to a structure.  Being a class adds an extra level of indirection.
[StructLayout(LayoutKind.Sequential)]  
class OSVERSIONINFO
{
    public uint dwOSVersionInfoSize;
    public uint dwMajorVersion;
    public uint dwMinorVersion;
    public uint dwBuildNumber;
    public uint dwPlatformId;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
    public string szCSDVersion;
    public Int16 wServicePackMajor;
    public Int16 wServicePackMinor;
    public Int16 wSuiteMask;
    public Byte wProductType;
    public Byte wReserved;
}

VB Definitions:

' Use this when the unmanaged API expects the structure passed by-value, or
' or if you want to pass it by-reference as a pointer to a structure
' If you plan to call "GetVersionExW" Use CharSet.Unicode
' If you are calling "GetVersionExA", Use CharSet.Ansi
  <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _
  Public Structure OSVERSIONINFO
    Public dwOSVersionInfoSize As Int32
    Public dwMajorVersion As Int32
    Public dwMinorVersion As Int32
    Public dwBuildNumber As Int32
    Public dwPlatformId As Int32
    <VBFixedString(128), MarshalAs( _
    UnmanagedType.ByValTStr, SizeConst:=128)> _
    Public szCSDVersion As String
    Public wServicePackMajor As Int16
    Public wServicePackMinor As Int16
    Public wSuiteMask As Int16
    Public wProductType As Byte
    Public wReserved As Byte
End Structure

' Use this when you want to pass it by-value even though the unmanaged API
' expects a pointer to a structure.  Being a class adds an extra level of indirection.
<StructLayout(LayoutKind.Sequential)> _
Class OSVERSIONINFO
    Public dwOSVersionInfoSize As Int32
    Public dwMajorVersion As Int32
    Public dwMinorVersion As Int32
    Public dwBuildNumber As Int32
    Public dwPlatformId As Int32
    <VBFixedString(128), MarshalAs( _
    UnmanagedType.ByValTStr, SizeConst:=128)> _
    Public szCSDVersion As String
    Public wServicePackMajor As Int16
    Public wServicePackMinor As Int16
    Public wSuiteMask As Int16
    Public wProductType As Byte
    Public wReserved As Byte
End Class