You need another laptop as well as the PPC.
Download and install: Auditor: http://www.remote-exploit.org/index.php/Main_Page
it is basically a boot from CD Linux with just the tools you need to 'probe' networks. use the PPC to find the Mac address, SSID and channel. Then fire up the laptop and start using all the tools to crack the WEP. The more data going over the network, the faster the apps will be able crack the WEP code. If enough data is flowing through the router, it is possbile to crack a 128 bit WEP in 10-20 minutes. If you have two laptops, you can get the other one to talk to the router (with spoofed MAC) to increase the data.
As already mentioned, all this depends if your card is supported by the apps, and can be put in silent mode (if you don't want to draw attention to yourself)
Of course, it is much easier just to walk 50 yards down the street and find an unsecured network for your free Skype calls.