Upload Windows Vista into its Cortical Processing Node. :p You'll find it stored in the invasive programs section of your tricorders electronic countermeasures database.

Though seriously, I'd second all the advice already listed. Nodes first, drones, infected, medical, tacs, elites.

With Elites, they can offload the energy damage they receive to other nearby drones, so for a speedy kill go melee on them. Proto drones can be one-shotted if you take them out as soon as they beam in.