Should I ice or heat a two week old knee sprain?

Whatever works... Usually i tell my patients to utilize whatever modality works for them.
Ice. Typically, within the first 6 weeks ice will help decrease inflammation. Often, in physical therapy heat is used to mobilize the joint, but then it is followed immediately by ice to diminish the swelling. The typical recommendation is rest, ice, compression and elevation for an acute injury. I would definitely consult with a physician to make sure there is no unsuspected fracture.