Catastrophic failure adding users

Hi

I have an error trying to modify file system permissions on my Vista

Ultimate X64 sp1 on which I am administrator. When I click "Add..." in the

permission dialog I obtain a box saying:

Title: Windows Security

Unable to display the user selection dialog.

Catastrophic failure

And I cannot add users.

If I use CACLS or similar tool I succeed with no problems. Any idea? This

also occurs trying to change permissions on other servers via shares.

This makes my system pretty unusable...

Thanks