[Gil]

Nationalism is more of a philosophical stance than a political one because it is more connected to cultural/ethnical roots than government of a nation. Correlation between nationalism and nacional-socialism/fascism is just the common-man's viewpoint, perpetuated by the biased, pro-immigrant minority media.
Nationalism is, if anything, the exultation of one's ethnic group and it's homeland and as such can be used by anyone who loves his nation and culture while a supporter of fascism can be pro-federal Europe or an imperialist.
On regards to communism, the "internationalist" spirit refers to political expansion of the communist doctrine, not multiculturalism. Alot of minorities were killed and shunned by communist Russia and China and "xenophobic" behaviour is not absent from communist states, the point is they want the whole world to be communist not to have a state composed of multiple ethnic groups.
Also communist can be nationalistic but of a centralizing stance since it supports the "common culture" and seeks to erase local/regional culture.
Anyway, and without boring you chaps anymore, the whole issue is roughly apart from economic/political/judicial ruling of a nation.