Don't use
Replay Media Catcher for Youtube. Use it for non-Youtube sites.
For Youtube, use the free version of YTD Video Downloader (YTD Basic) from
http://www.youtubedownloadersite.com
Be careful when installing it. It tries to add crap to your computer, so you have to uncheck/un-X boxes for toolbars, adware, payware, etc. I forget what all it tries to install. Again, read very carefully, because lots of these little two-bit software companies (usually individuals) try to trick you.
It only works for Youtube.
I find this to be a better alternative to keepvid.com, which has too many Java problems these days.