Well, my experience of trying to sniff or pentest under Vista is null. You are going to need to be able to put your NIC into permiscous mode or monitor mode. Cain and Able can do that if your NIC supports it. Not sure if C&A is supported under Vista though, and I don't want to go look at the moment. You can also packet capture with either Wireshark or Kismet. Just do a search on insecure.org of wireless sniffers and you'll see what the most popular ones are. If you want to really get into learning sniffing, or pentesting in general, I would suggest a different OS than Vista... preferably Linux, but definately not Vista. All you will be able to capture is what traffic they are sending out, or possibly recieving on their computer, not specifically what they are viewing unless you open a connection to their computer with a remote desktop or vnc type connection via an exploit. Or... go manually to whatever pages they are on. Aircrack is a program that cracks WEP and WPA AP's after it's been able to sniff out an encrypted key from the 'ether'.
"Bad.. Good? I'm the guy with the gun"