Try this one.
It’s freeware, compact and works well, and also includes a user-settable auto-update feature. However, be aware that in the past they included other promo stuff in the installation, so you have to pay attention during the setup process.
Also, after installation, check that the downloader was the only thing that was installed. You can do this by clicking Start → Control Panel → Programs and Features. In the Programs and Features console click on the “Installed On” header to sort the list by install date. The YouTube downloader should be at the top. If anything else was also installed that you don’t want, you can uninstall it by right-clicking on it and selecting “Unininstall”.