You don't need conduit, although if you're running it through something like a cold air return I believe you need Plenum rated cable. For both my current and previous house I used existing in-wall runs of coaxial and/or telephone lines to run ethernet. Just pulled the cover off, cut the cable, taped the ethernet to it and went into the basement to pull it through. Mind you, I didn't have a lot of bends to deal with, and we ditched landline and cable years ago so we didn't care about the existing lines.