Uhhhhh... Help? iOS 5.1.1 Installing Issue

I have a strange glitch where I can't update to iOS 5.1.1.
When I click the download, it brings up the apple terms and conditions, and I click agree. When I click agree, it says "An error occured when installing iOS 5.1.1". It then asks me to go to settings, or cancel. No matter which one I press, the 'Install Now' button is grayed out. I go back and click software update again, the whole same series of things happen again. If my iPad is plugged in or is at 50% or greater (or both), it will do the same thing. I've tried this over lots of times, for months and it still doesn't work. Can anyone help?

-Wrote on my iPad 2