I have had a headache for a week and a half behind my eyes. I have sinus congestion but no drainage or fever. Is this caused by a virus?

Maybe. Your headache very likely is a consequence of your sinus congestion, even without drainage. It can be from allergic reaction to inhaled allergen like pollen, a viral infection or even a bacterial infection. However, you may be right that this is not from a bacterial infection as you are not systemically sicker (fever, chills, swelling around the facial area). Try otc anti-allergy/decongestant.
Family practice. Headache lasting that long should be evaluated. It could be sinus pressure , allergies , or infection from viruses and bacteria. But it could be sign of something even more serious.