On windows 7, you can just select your two network devices and then right click on one of them and go bridge connections.

Make sure your xbox 360 is off when you do this, and that you are connected to the internet with your wireless. Then you can turn on your xbox and configure the network settings in it's control panel.
IP Address: Give it an IP address that would be on your network. For example, if your computer is 192.168.1.10, you could give your xbox the IP of something a bit higher that wont be assigned by DHCP like 192.168.1.150
Subnet Mask: 255.255.255.0
Gateway: Your Routers IP (likely 192.168.1.1 or 192.168.0.1)
DNS Server: Enter your ISPs DNS servers here or OpenDNS servers (208.67.222.222 and 208.67.220.220)
After you set that up, in your router's control panel, you can forward port 88 and 3074 to the IP address you gave your xbox in order to give it an open NAT so you have no problems with hosting games.
The only problem I've encountered with this is that you have to make sure that you are always connected to your wireless internet before turning on your xbox or else it will think your xbox is an internet connection and you'll likely have to restart your computer to get it to start working again.