Once as many IVs as required have been captured using the airodump packet capture utility the resultant [filename].cap file can be imported into aircrack to break the static WEP or WPA-PSK keys.
To view all available switches:
cd c:\aircrack-ng-[version]aircrack-ng.exe
Basic usage:
aircrack -q -n [WEP key length] -b [BSSID] [filename].cap
Breaking WEP:
As you can see the capture file in use below (capture1.ivs) was created with airodump capture option "Only capture WEP IVs (y/n)" set to "y" as the resultant file is in the .ivs format. If n had been selected the resultant file would be in the .cap format.
Whilst this will work, we can could have cut down the cracking options for Aircrack-ng...