Judging by what you've already written, I guess you've already found the 'Ease of Access' centre in the control panel? Your going to have to go through it all again and return it back to it's basic settings. If this is failing to work for some reason, you could try creating another user account to sign in with and deleting the old one.
Regarding the desktop, check the settings found via 'Personalize' when you right-click on the desktop. If you look under 'Display settings' or 'Themes' you may find the answer there.
You could also try 'System Restore' if all else fails.

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