Actually, the attackers were shielded, I guess you didn't watch the movie enough.
As for why? It's Hollywood, man. And you have to admit that a buttload of seventeen mile wide ships coming out of space and parking over our cities would be pretty damn scary.
The resources thing...probably means raw materials. Getting us out of the way so they can use up the arable land and potable water and excavate whatever precious metals and such that they use in their technology.
The books I've read expand on it a lot; they use an awful lot of organic technology in their civilization and they spread their power out through their various ships.
As for why they've never heard of a computer virus, well, I don't think they haven't, but it's like if you took humans from 200 years from now and reintroduced polio. Sure, you've got advanced technology capable of defeating it but first you've got to figure out what the hell it is and that takes time.
The movie dialogue specifically states, while not directly, that David Levinson suspects that once they identify the infection they'll be able to excise it easily, hence there being a window of "a few minutes" during which the energy shields were down.
But on that note, he demonstrated that the shields were up on the attacker they had in area 51, but he walked through them easily to put a Coke can on a ledge inside the shield.
That said, why the hell didn't they just get a bunch of sky cranes and lower nukes down onto the surface of the destroyers instead? With the shields still up it would've reflected the force of the nuclear blast back down into the ships, further destroying them.
Sure, then you've got a shitload of irradiated alien stuff sitting around, but it beats the hell out of doing something stupid like sending a single ship up to their interstellar hive and hoping that your communications expert can break into their network and upload a virus that may not even work.
EDIT: So, to clarify, they're telepathic, they organize into a hive mentality, they are divided into two subspecies (one a controller race and one a warrior race), and they use organic equivalents whenever possible. So it's not too much of a stretch to think that they don't have guided missiles or targeting computers, given that (according to the novels anyway) they have to have three warriors in each ship just to make it fly straight let alone shoot.