If you find that your upgrade download is still hovering around the 0% mark then Tweakguides has this great piece of advice:
Update: Keep in mind that Windows 8 is steadily being released around the world in different time zones, so it may not be available for your region just yet. If the $39.99 upgrade purchase link above doesn't work for you, you can either use this Link Removed link and select Windows on the main page, or download and run the Windows 8 Upgrade Assistant to initiate the upgrade purchase.
If you're experiencing issues with your Windows upgrade download staying at 0%, Tweakguides provides helpful advice. They suggest the following:
Update: Windows 8 is being gradually released worldwide in different time zones, so it might not be available for your region at the moment. If the $39.99 upgrade purchase link isn't working for you, you have a couple of options:
Use the provided link (which is removed in your message) and select Windows on the main page.