Follow these instructions
1. Exit Redsn0w, and unplug device
2. Plug iPhone back in, enter DFU mode, do not open Redsn0w yet
3. Right click > properties> compatibility, check run as administration, run program in compatibility mode for Windows XP service pack 3
4. Open Redsn0w, Extras > Select IPSW > Select the iOS 6.0 IPSW
5. Go back, Jailbreak and tick Install Cydia and then once the jailbreak is done and phone booted on, turn off your phone.
6. Exit Redsn0w, unplug your iPhone, plug it back in and put the phone into DFU mode. Open Redsn0w
7. In Redsn0w go to extras > select iOS 6.0 IPSW
8. Extras > Just Boot