"Attempted to read or write protected memory" with Winscard.dll

[nieve]

Hello all,

I'm working on a c# application using smart cards.
I've been using vs2008 on an XP machine and everything was ok till I tried installing and running a demo on a vista machine. What happens is that when I get to call this method:
[DllImport("WinScard.dll")]
public static extern int SCardListReaderGroups(int hContext, ref string cGroups, ref int nStringSize);
I get the "Attempted to read or write protected memory" exception.
I've tried editbin.exe /NXCOMPAT:NO myDllPath (as suggested here ) but that didn't help.

Any ideas or suggestions would be highly appreciated!!