I have a laptop with win 7 64 bit. I have used greenpois0n RC5 to JB my iphone 4 running 4.2.1 with no problem on the first try. I have since tried to do 2 friends iphone 4's with 4.2.1 and have had no luck. Both are new phones on ATT. I have restored them back to stock 4.2.1 and then retried with no luck. I installed a random program from phone and synced, installed different program from itunes and synced again. Still no luck. I am running program as admin and tried compatibility for XP with SP2 and SP3. I used GP5 and also tried GP6 when it came out as well. I also tried different laptop with XP 32 bit and same problem. I get into DFU mode just fine and it gives me the jailbreak button. I click the button and in about 15 seconds or so it just says JB failed, try again.
I then read that some people used redsn0w and then put GP over it. I was able to get redsn0w to work with no problem, but it still would not load GP on top of this, so the JB is still tethered but working. I am scratching my head trying to figure this out. On GP it gives the countdown timer and the jailbreak button always appears before the timer is down to zero so I haved tried to wait until it gets to zero and all times after it makes the USB sound. Still no luck. Any help appreciated. I have also done my wife's 3gs using GP5 and had no problem. Took a couple of tries, but works fine.

Brian