Under extreme circumstances, it is sometimes necessary to edit your PC's registry. The most common cause of this problem is the presence of older Xilinx software on your system. (e.g., XACT, Foundation 1.5i, 2.1i, etc.) Older versions of Xilinx software sometimes pose conflicts with newer versions.
The steps to edit the registry are:
1. Go to Start -> Run. Type "regedit" and press <ENTER>.
2. Use Edit -> Find to search for anything relating to Xilinx or Synopsys. A few useful search parameters are "Xilinx", "3.1i", "2.1i", "1.5i", "FPGA", "XACT", and "Synopsys".
3. Delete anything that is unambiguously related to Xilinx or Synopsys's FPGA Express software.
WARNING: Deleting other entries can lead to serious system problems.
4. Restart the computer.