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 int dwOSVersionInfoSize;  
   public int dwMajorVersion;  
   public int dwMinorVersion;  
   public int dwBuildNumber;  
   public int dwPlatformId;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)]
   public string szCSDVersion;
}

// 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 int dwOSVersionInfoSize;  
   public int dwMajorVersion;  
   public int dwMinorVersion;  
   public int dwBuildNumber;  
   public int dwPlatformId;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)]
   public string szCSDVersion;
}

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
  <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _
  Public Structure OSVERSIONINFO
   Dim dwOSVersionInfoSize As Integer
   Dim dwMajorVersion As Integer
   Dim dwMinorVersion As Integer
   Dim dwBuildNumber As Integer
   Dim dwPlatformId As Integer
   <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> _
   Dim szCSDVersion As String
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
   Dim dwOSVersionInfoSize As Integer
   Dim dwMajorVersion As Integer
   Dim dwMinorVersion As Integer
   Dim dwBuildNumber As Integer
   Dim dwPlatformId As Integer
   <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> _
   Dim szCSDVersion As String
End Class

Notes:

The C# signatures are incomplete, but I've only tested the VB ones.