I'm not a Steam hater, or anything like that, but for modding purposes, and convenience purposes, a lot of people don't want the Steam version of Warband, which is what comes with the CD.
So here's how to install it without Steam.
1. Download Steam: http://cdn.steampowered.com/download/SteamInstall.msi
Make sure you save the setup file! You will need it later!
2. Follow through with the installation. Log in, or create an account if you don't have one.
3. Get your Mount&Blade Warband box. Click "Add game" down on the bottom left of the Steam window. Enter your Warband serial key that is on the box. Download and install, or install it from the CD. Your choice.
4. Right click on your taskbar. Select "Task Manager" Select "New Task" type "regedit"
5. You will see a list of folders. Select "HKEY_CURRENT_USER" Then select "Software" and then scroll down until you find "MountandBladeWarbandKeys" Select that. On the right is a key. Save this somewhere. An alternate way of doing this is by pressing "Ctrl+F" and searching for "Warband"
6. Run the Steam setup again, the one you saved from step 1. Follow the prompts and it should un-install just fine.
7. Go to www.taleworlds.com and download Warband. Install and follow the prompts.
8. Open the Launcher. If it allows you to activate, enter the key you saved. If not, then it found it already in the registry. Voila! You're finished!
This should also work for the original M&B, if you bought it from Steam, but want to install mods easier etc.