1. exit Redsn0w, and unplug device
2. plug device back in, enter DFU mode, do not open Redsn0w yet.
3. if have not done extract Redsn0w file to a location such as desktop.
4. right click/ properties/ compatibility, check run as administration service pack 3
5. open Redsn0w/ Extras –>Select IPSW...