Alias is very different from La Femme Nikita.
La Femme Nikita (1997) is a Canadian TV version of the French film La Femme Nikita (1990), and in fact there is now a TV adaptation of the French film being shown on CW called Nikita (2010) - whit btw is very good. There is actually an American film adaptation of the French film called Point of No Return (1993) but it's so-so.
Alias and Nikita has a completely different storyline.
In Nikita, the protagonist is a woman. She used to be a criminal and was forced to become a spy / assassin by an organization called the Division. In this story, Nikita goes rogue and tries to defeat the organization.
In Alias, the protagonist, Sydney Bristow, is also a female spy - but she does it willingly. There is some twist in the plot about her relationship with the CIA (I don't want to give any spoilers in case you haven't watched it), but ultimately there is some very interesting sci-fi-ish spice that is not seen elsewhere on television.
You should take a look at this series. Alias is on my all-time top 10 tv series. I've watched complete seasons twice!