you're going to have to re-encode your footage.
if you don't have a lot of space, I reccomend making small clips in virtualdub/mod and re-encoding those instead of entire episodes.
you should use a lossless codec like lagarith or huffyuv
and you really should buy the dvds (if avaliable in your area), you'll get better quality and some other good things too
links (if you don't already have them)