Which model of Xilinx FPGA are you implementing a design. Current ISE Webpack only support a limited number of model versions. You may have to download a previous version from their archives.
Here's the product support matrix for Webpack ISE 12 and Webpacks ISE 10.1 and earlier. I'm have trouble finding Webpack ISE 11.x, I'll keep looking. I can tell you from experience that for Virtex 2 Pro you'll have to use v10.1, v9 or v8. The product support is also limited by the size of the FPGA, so look carefully for your exact FPGA model.