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10.1 ISE - Project Navigator freezes if I select File -> Open and select an NGC or NGR file
To avoid this problem, select the top-level source file and then run the "View RTL Schematic" or "View Technology Schematic" process from the Processes window.
This problem has been fixed in the latest 10.1 Service Pack available at:
The first service pack containing the fix is 10.1 Service Pack 1.