Running on Vista64 fine here. I was able to create ipsw. Using expert & custom mode.
The way I can make/reproduce the crash is to change the partition size from 700 to 500 or 600. As soon I make that change, winpwn 2.5 will crash
Note: The minimum default is 700 MB, you cannot change it to a smaller number or winpwn 2.5 will crash. It is ok to make it bigger but not smaller.