Project Navigator 9.1i will fail to run implementation in Xplorer mode if there are spaces in the ISE project directory path.
If the design is placed where the path does not have a space in it, Xplorer will run without any error.
This problem has been fixed in the latest 9.1i Service Pack available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jspThe first service pack containing the fix is 9.1i Service Pack 1.