Is only Coregen broken? Do other Xilinx tools work ok?
I'm not sure, but here are a few simple guesses:
1. Install both ISE CDs.
2. When you install ISE, enable the installation option that lets it update your path and environment variable.
3. If you run the Xilinx tools from the command line, restart the Command Prompt window after ISE installation, so the environment changes can take effect.
4. Maybe another program in your system has the same name as one of the ISE tools that coregen.bat is trying to run. You could try rearranging the items in your path variable.