In What Can I Do About The End of XP Support you say to run Windows 8 Upgrade Assistant. You can’t run Upgrade Assistant if you are running Vista or XP. You can only run it if you are running Windows 7.
Windows 8 Upgrade Assistant should work with XP. Make sure you are running Windows 8 Upgrade Assistant  and not Windows 8.1 Upgrade Assistant. They are featured on the same download page from Microsoft.
The 8.1 Upgrade Assistant  will not run with XP. However, 8.1 and have very similar requirements, so if your computer will work with 8, it will probably work with 8.1.