It's not so difficult: install one of the two, either winzip or winrar (I'd recommend winrar, but after a while it will be bugging you to buy, you can still use it though). You do this with a double click on the installer (the file you downloaded). The install wizard will guide you through the installation process.
After you installed the program, the packaged files you downloaded will show the icon that is associated with winrar. To install these packaged files, just double click, a window pops open and you can drag and drop the .package files to your downloads directory (..\My Files\EA Games\The Sims 2\Downloads - it's the directory, where the neighborhoods and all the user data is located, not the installation path). If you don't have that directory yet, just create a folder called "downloads" and put your package files in there. If you plan on installing more content in the future, which is likely, because it can easily become an addiction once you see how beautifully you can enhance your game with downloads, you should sort your stuff. There are good tutorials on MTS2 how to do it. You need to have at least Nightlife to use subfolders.
The clean installer is for installing the Sims2Pack files (Sims) and for organizing your downloads folder.
If this was confusing, just ask again.
