Here are the steps to get the ISE Webpack working in Windows 8:
1. Download the one .tar file and the three .zip.xz files from: http://www.xilinx.com/support/download/index.html/content/xilinx/en/downloadNav/design-tools.html
2. In order to verify the integrity of the download, get an MD5 checker and use the MD5 SUM values under the download links to check that you downloaded the files correctly. A good one is http://winmd5.com/
3. Once you've verified that your files aren't corrupted (apparently it's pretty common to have issues regarding that), extract the .tar and install Xilinx as normal. Be sure to just install the Webpack (the free one)!
4. Assuming you've installed Xilinx into the default directory (C:\Xilinx), go to 'C:\Xilinx\14.7\ISE_DS\ISE\lib\nt64' and type in your search bar "libPortability.dll". Rename "libPortability.dll" to "libPortability.dll.orig".
5. Next, in the same directory, make a copy of "libPortabilityNOSH.dll" and rename the copy to "libPortability.dll".
6. You should now have three "libPortability*" files: "libPortability.dll", "libPortabilityNOSH.dll", and "libPortability.dll.orig".
7. Now, copy the file libPortabilityNOSH.dll from "C:\Xilinx\14.7\ISE_DS\ISE\lib\nt64" into "C:\Xilinx\14.7\ISE_DS\common\lib\nt64". Now do this to this new directory: rename "libPortability.dll" to "libPortability.dll.orig", and rename "libPortabilityNOSH.dll" to "libPortability.dll".
8. You're done. Now, Xilinx should be just as stable as it is in Windows 7, and the Xilinx License Configuration Manager won't crash every time you try to open a license file. You can also now save projects.
9. If this doesn't work for you, it's likely that the 32-bit version of the executables are more stable. If you want to go 32-bit, you should right-click on the Xilinx shortcut on your desktop and go to "Target" field. Go to the end of that long path, and at the end it should read "...\ISE\bin\nt64\ise.exe". You can change the "nt64" to just "nt", and Xilinx will be more stable than ever. Obviously only do this if you have issues though.