Hi there. Here I am, after three system restorings thanks to Decor8.
I know I'm not the first one to have this problem, but I'll try to be the one who explains it better...
My Decor8 trial period expired and I decided to uninstall it (it worked perfect all the time, but I can't buy it right now). All the uninstalling process goes well, no problems. If I'm not wrong, after the uninstalling the program asks you to reboot Windows, so I reboot it.
After rebooting Windows, this is how my start screen looks like:

The easyest way to describe it is that looks like that Decor8 deleted the Star Menu backgrounds (or disabled them, I have no idea), so the Start Menu and the desktop show together.
The start menu is usable, but the icons have the annoying effect that leave images where they go through:

If you go to the desktop, you can do nothing but moving the mouse. The icons seem to respond but take a loooong time to act (for example the window that appears on the captures), so you can do nothing. Nothing appears if you right-click on something. Ah, and the desktop background disappears too.
So, basically, all Windows 8 gets messed up and unable to be used.
¿Solution? I did a system restore. After it, everything went back as before uninstalling Decor8:

At this point, as you can see on the pic above, you find that because the trial expiration, the background on the start screen is just a plain colour, there's no drawing scheme. So, without uninstalling Decor8, I go to the customization options of Windows 8, and try to change this (I want at least those ugly drawings that the Start Screen had, not a plain colour!), but it doesn't work, you can change any colour you want, but you can't select a drawing scheme. I mean, you can select any of them, but nothing happens, the Start Screen continues having the plain colour you selected as background.
So I decided to try uninstalling Decor8 again.
This time before uninstalling it, I installed the program again, just to prevent any problem that the program could had any problem that cause all this mess when uninstalling it. So after installing it and rebooting, I uninstalled again Decor 8. And the mess gets back again. =____=
So... I did a system restore again. And we get back to the beginning.
I looked for some info about this, and thats when I saw I'm not the only one with this problem (you have some examples even in this forum: https://forums.stardock.com/438361).
One interesting option somebody said in another forum was this:
Right click on My Computer, Manage, Service and Applications, Services, right click stardock Start8, properties, Start Type (set it to disabled).
I tried this, and fter rebooting the result was exactly the same as uninstalling the program.
So you know what? I had to do a system restore again! 
And here I am, without being able to uninstall Decor8, wich remembers me that the trial period expired every time I turn on Windows 8, and without being able to do anything on the background of the Start Screen except changing its colour.
My configuration:
- PC: Asus S1 (Atom dual core, Nvidia ION II, 4 GB RAM).
- OS: Windows 8 Pro Media Center (64 bit).
- Complementary customization programs: Oblytile: allows to change the Start Screen Icons (http://forum.xda-developers.com/showthread.php?t=1899865) , Classic Shell: a free alternative to Start8 (http://classicshell.sourceforge.net/). But as I said, even using these programs I had no problems with Decor 8.
So here I am, asking for a solution for this.