There are many users already moving from Windows 7 to Windows 8 using the upgrade path, and even though upgrading should be a straightforward process, sometimes is not the case. For example, there are users that have purchased Windows 8 upgrade online, but during the install they are getting stuck at 90%. No just that but also the installation fails after hours of waiting.
Now the solution to this problem will depend on many factors (hardware configuration and conflictive software), however, there are people that have found a fix (or workaround) to this issue. So if you find yourself in this situation, this is how you should go troubleshooting:
Disable all unnecessary drivers. This also include network, sound and video drivers. Also (and really important) if you using the Windows 8 upgrade assistant, opt creating a DVD or USB out of the installation files (ISO) — Here is how.
Furthermore, whenever possible, do a backup of all your documents and files and choose no save anything during the install. In other words, do a clean install — and yes you can install Windows 8 from scratch using the upgrade media of Windows 8.
If you still getting stuck at 90% (or any other %) of the installation, you may have a media installation that is corrupted, try to download the ISO file once again.