Well, he said he's "stupid" with computers, so I'll do this step-by-step.
1.)Go to http://www.7zip.com/ and download 7zip. Install 7zip. Download your mod, if it is a .rar, or .zip file, open with 7zip.
2.)After you open the file with 7zip, go to your desktop, or wherever you keep your M&B Warband Icon, right click it, and go to properties.
3.)Depending on if you have steam, or you bought the game directly from taleworlds, this step may differ. (Non Steam) If you have windows vista, or windows 7, it should have a little button that says "Open file location". If there is a button, click it. If you have windows xp, right click, and go to properties. Copy the line of text, it should be something like "C:\Program Files\Mount&Blade Warband\". Paste this in your search bar in the start menu, and hit enter. It should take you to the root folder of your mount and blade game. (I won't cover steam)
4.) Go back to 7zip, and drag and drop the folder (it should be named after the mod you want) into the "MODULES" folder, it should be a subsection in the root folder.
5.) Wait for it to finish copying.
6.) Open M&B Warband.
7.) Before you click "Play", click next to "Current Module" and scroll down to your mod.
8.) I advise you copy this down to a text file called "M&B MOD INSTALLATIONS", or bookmark this page so you don't forget.
9.) Play.