Web server upgraded to 2003 & IIS 6 and now call to win32 dll will not work

[Tom]

We just upgraded our web server from Win2k and IIS 5 to Win 2003 .Net
and IIS6. I have an asp.net application that calls a win32 dll using
dll import. It worked fine on our old server. When we moved
everything to the new server I get the following error:

Object reference not set to an instance of an object.
....

It occurs when I try to use the following:
[DllImport("fcregcw.dll", CharSet=CharSet.Auto)]
public static extern void MakeRegistration(ref Tag_RegStruct RegInfo,
[MarshalAs(UnmanagedType.LPStr)] string pPassWord);

What has changed with using win32 dlls and the new server? Any ideas?

Thanks,
Tom