If you've watched them since last cleaning out your temp files, you may be able to go in there and find that they are still on your computer (which is why they say to delete temp files on a reg basis, so that your computer doesn't get bogged down). If they are still there, create a new folder in under Videos and then move the temp video file to the new folder. Depending on the source/format, it's then saved on your hard drive (or thumb drives as the case may be) so that you can watch it whenever you want. I've successfully done this with various videos/news clips.
There are also supposed to be some free on-line programs (I believe one is called KeepVid or something similar) that are supposed to enable the saving of youtube videos to your computer...
Like with River Cottage (I love Hugh!

)...I'm planning on getting the complete boxed DVD set. Granted, I can't watch either of them them on my regular DVD player since I don't have a region-free player (US is Region 1, while the UK is Region 2, and they are not compatible

), but I
can, however, play them via my computer's DVD player. So I simply hook my computer up to my TV and watch them on the television screen.