Hi, Forewarning to those who have yet to install 8.1i and who use core generator. Hard to believe I know, but don't use a space in your 8.1i install path otherwise you _may_ have problems running core generator. (I certainly did.).
Here is the relevant answer record, scroll down to solution 2.
"Solution 2: In the ISE 8.1i design tools, this error message might occur if there is a space in the Xilinx install path"
Regards Andrew