Entering "impact" on the command line, selecting the "impact.exe" icon, or invoking iMPACT using the Start -> Program menu does not run the iMPACT application.
To work around this issue, open a project in ISE and select Configure Device (iMPACT) under the Generate Programming File process.
This problem has been fixed in the latest 9.1i Service Pack available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is 9.1i Service Pack 1.