Xilinx ISE 14.7 installation instructions for Windows 10

You must install Xilinx ISE 14.7 (October 2013) to target the Spartan-3A FPGA. It is available at:
https://www.xilinx.com/support/download/index.html/content/xilinx/en/downloadNav/design-tools/v2012_4---14_7.html
(Note: Xilinx ISE 14.7 from December 2017 does NOT support the Spartan-3A) 

To run the October 2013 release on Windows 10: install as usual, then do the following:

1. Navigate to the following ISE install directory: <install_path>\Xilinx\14.7\ISE_DS\ISE\lib\nt64\

2. Rename the file "libPortability.dll" to "libPortability.dll.orig".

3. Copy the "libPortabilityNOSH.dll" file to the same folder, and rename it to "libPortability.dll".

4. Repeat steps 1-3 in the following folder: <install_path>\Xilinx\14.7\ISE_DS\common\lib\nt64\

The above steps substitute the original "libPortability.dll" with a "libPortability.dll" file that has SmartHeap disabled, the NOSmartHeap (NOSH) version. This does not negatively impact the operation of the tools, and should successfully work around the ISE 14.7 crash documented above.

See Xilinx AR# 62380 for more details:
https://www.xilinx.com/support/answers/62380.html