What do you mean exactly by "using it over the LAN?" There are different ways to use a webcam, but I don't think any are used strictly over a LAN. You could download a webcam messaging service like Eyeball Chat (
http://www.eyeballchat.com) or Yahoo! Messenger (
http://www.yahoo.com). I've found out through experience that Eyeball Chat works the best with webcams. It's also a great way to send files to people that have firewall issues over AIM and Yahoo!. Just remember that in order for someone to see through your webcam, they have to have the program installed as well.
Windows XP has a webcam program built in, but I don't think it's possible to share it over a LAN, or how that would work even if it were possible. The XP program does take fairly decent pictures though. Good luck!
-Lasereth