Salander's computer hacking skills are extraordinary. The techie geek/punk is a common feature of popular fiction and movies. Skilled kids are thus able face off those in power and authority. Salander is squarely in this lineage.
Other top techie punks are:
David Lightman, WarGames
Neo, The Matrix
These fictional characters are not without real life exemplars. In 2008 Gary McKinnon of Glasgow was charged with hacking into the US Pentagon computers. He suffers from Asperger syndrome, a condition Blomkvist suspected Salander of having in the previous novel.