Unable to read diskvol serial ID under NT. Works with D3 and 95/98

Can anyone help please,

This function works fine under windows 95 and 98, but not with NT4. Any
clues?

function TMainForm.GetDiskVolSerialID( cDriveName : char ) : DWord;
var
  dwTemp1,
  dwTemp2 : DWord;
begin
  GetVolumeInformation(PChar( cDriveName +
':\' ),Nil,0,@Result,dwTemp2,dwTemp2,Nil,0);
end;

Function is called like this

   SerialID:=Format( '%X', [ GetDiskVolSerialID(CDDrive) ] );

where CDDrive is a char holding the drive letter.

TIA

Jeff