I've heard it is possible to use a Windows Mobile phone as a wireless modem for a Windows system via USB, Serial, Bluetooth, and/or IrDA. Read here:
http://windowsmobile.msmobiles.com/?p=10
Also, if you can't locate your Windows Mobile CD, you can get USBmodem_dialer.exe here (at the bottom of the page):
http://sx66-blueangel.home.comcast.net/Extended.htm
Note: You'll need to have the correct login information (e.g. username, password, etc.) for your service provider (e.g. Cingular) for this to work. You should be able to get this information by inspecting your connection settings or from your provider's website.
BTW - I'm not sure if there is anything similar for WiFi but I'll post here if I find anything. Good luck.