ladda ner is a particle verb, so the stress always goes on ner'. It's a question, so the verb must go before the subject, and the question word goes first of all. But verb and its particle are easily separated (the subject easily goes between them, and adverbs too, but not objects). So the particle goes last. If you'd add an object instead, it would go after the particle: Laddar du ner en fil?* 'Are you downloading a file?' but the particle would still go after the subject.
I was wondering, I see both "hämta" and "ladda ned" on the internet for downloading files and loading pages and such. What would you say is the difference between the meanings of these two words as far as it relates to use on the internet and with computer related things?