Economical external HD for back-up and software.

If I can't find a decent laptop (see my other posts), I will want to back-up
my HD and clear out all the Toshiba gook, but maintain my download games
from streaming sites. So I would want something that allows me to pick and choose
what I want to keep. I don't have much on my HD (about 100GB), so I don't need
a large HD. But I don't know much about this routine. Maybe back-up HD's come
with software for back-ups. I want a clean HD to take advantage of MS Win8 offer.
(presently have Win7 Home. Hmm, just realized I'd have to have Win7 left on for
upgrade, right?)

Thanks for the help.