Note:Yes there is a windows version, but it's really not worth it as the wireless drivers for windows hardly ever allow for monitor mode on wireless devices
Aircrack-ng is an 802.11 WEP and WPA-PSK keys cracking program that can recover keys once enough data packets have been captured. It implements the standard FMS attack along with some optimizations like KoreK attacks, as well as the all-new PTW attack, thus making the attack much faster compared to other WEP cracking tools.
In fact, Aircrack-ng is a set of tools for auditing wireless networks.
Tutorial for WEP:
A handy little tool to simplify the use of the Aircrack-ng suite (as if that is needed)
Tutorial for Airoscript: