There are two sites that I know of that have the full episodes online. The quality isn't that fantastic but it isn't bad either.

www.tudou.com
www.youku.com

Just type Roswell in English and it will bring up a list of vids that you can watch. Here is a link to one episode as an example of the quality.

http://www.tudou.com/programs/view/rNcxdUseQ-M/

season one, in excellent quality is on hulu.com