I made a very stupid mistake that took me 3 days to figure out why I was getting error 6. The standard answer on here is not being in DFU mode.
There is another reason. I stupidly had my iPhone (version 1) updated to the firmware that is for 3G, that is: iPhone1,2_2.0_5A347_Restore.ipsw
Pwning that firmware won't work on original iPhone.
The firmware you need is iPhone1,1_2.0_5A347_Restore.ipsw
One tipoff this may be your case, is that when you are in the Winpwn
program, it won't let you add the bootloaders.
Lesson learned the hard way, but now it worked !