Erase a 1st gen iPad
The iPad 1 should be good to go for a valid version of 5.1.1


http://www.icj.me/ios/all#iPad11


I'd say the easiest thing to do would be to take the iPad to a copy of iTunes that has a good internet connection or download a reputable copy (for whatever reputable means to you) of 5.1.1 IPSW and use your iTunes to wipe and restore the iPad.

Jailbreaking might be an option, but I'd go with easy.