Looks like I found a possible cause of the problem: http://msdn.microsoft.com/en-us/library/ms723664(VS.85).aspx
A little bit down on that site it says this:
Bugs exposed by this property:
Windows Vista and Windows 7: ** NOTE ** THIS PROPERTY IS ACTUALLY WRITE-ONLY. IF YOU ATTEMPT TO GET ITS VALUE, AN ERROR WILL BE THROWN.
So eventhough the documentation tells us the property is read/write there seems to be a bug under Vista and Weven that makes it write only.
I hope that explains your problem.