codec -> COmpressor/DECompressor
so whatever codec did the compressing will most likely also be doing the decompressing (which happens when you play the video on your computer)
methinks that you probably are looking to improve video quality or re-encode footage so you can edit it.
for the first, I suggest avisynth (along with virtualDubMod and avsp)
for the second, I suggest
making clips with vdubmod and using a lossless codec such as lagarith or huffyuv