rDownloader is only 600KB. It works seamlessly on all Windows versions..

Rdownloader

We recently launched Rdownloader a free youtube downloader, while the market for such programs is saturated we felt that there was not a program that offered the complete package for free, either limited programs or limited versions demanding you buy the premium version in order to enjoy the full capability of the program. We therefore decided that we would not have a premium version for this software rather we would offer all possible features in the free version, faster downloads, multiple simultaneous downloads, multiple formats, and even downloading from multiple websites such as daily motion and metacafe.

we are still expanding the services but are pretty proud of the first version. So its official we have stepped into the deep end of the Free Youtube downloaders pool and so far we are swimming pretty nicely. We hope to launch our next version in the next 3 months any feedback you would be awesome.