10/8/15: This guide will also work for Windows 10 64-bit 
I recently scored a Spartan 3E Starter Board on eBay. So, thinking I was on to a winner as I used this board during my time at uni, I downloaded and installed Xilinx ISE WebPACK.
But the problem is, it barely works in 64-bit mode on Windows 8

The license manager and Project Navigator both just close when you try to open a file and PlanAhead only works in 32-bit mode.
Xilinx ISE does not officially support Windows 8 and it probably never will as Xilinx are focused on their new suite - Vivado. However, Vivado only works with 7 series devices and upwards, so no Spartan 3 or Spartan 6 FPGAs!
But all is not lost, here is how you can get ISE (64-bit) working on Windows 8.
Fixing Project Navigator, iMPACT and License Manager
Note: I am assuming you are using ISE 14.7 and have installed it to the default location
- Open the following directory: C:\Xilinx\14.7\ISE_DS\ISE\lib\nt64
- Find and rename libPortability.dll to libPortability.dll.orig
- Make a copy of libPortabilityNOSH.dll (copy and paste it to the same directory) and rename it libPortability.dll
- Copy libPortabilityNOSH.dll again, but this time navigate to C:\Xilinx\14.7\ISE_DS\common\lib\nt64 and paste it there
- In C:\Xilinx\14.7\ISE_DS\common\lib\nt64 Find and rename libPortability.dll to libPortability.dll.orig
- Rename libPortabilityNOSH.dll to libPortability.dll
Fixing PlanAhead not opening from 64-bit Project Navigator
PlanAhead will not open when you are running 64-bit Project Navigator (e.g. for I/O Pin Planning), it just displays the splash screen but never opens.
To fix it, we have to force PlanAhead to always run in 32-bit mode.
- Open C:\Xilinx\14.7\ISE_DS\PlanAhead\bin and rename rdiArgs.bat to rdiArgs.bat.orig
- Download the attached zip file
- Extract it. You should now have a file named rdiArgs.bat
- Copy the new rdiArgs.bat file to C:\Xilinx\14.7\ISE_DS\PlanAhead\bin
Now you should have a working ISE Design Suite on Windows 8 64-bit
Hope this helps!