Your taskbar isn't actually "Dimming", it's actually a transparancy overly, from what appears to be a task starting on boot. Your image shows the string "SEO Doctor". This is a plugin for firefox. If you aren't using it, try uninstalling this plugin from Add/Remove Programs and see if the issue goes away. You are seeing the bottom part of a window in "transparency mode". Some people like this kind of eye candy effect, personally I find it confusing and annoying to see through a window to the windows underneath.
Edited by Gabrial, 12 October 2010 - 03:09 PM.