You could just have sinusitis, which doesn't necessarily indicate an infection but just inflamed sinus passages due to dry air, allergens, a previous cold, etc.
My doctor, before presribing antibiotics, will always push me to try to take Sudafed (and NOT the PE kind) on a regular basis for a week or so. This gives the decongestant enough time to truly "loosen" things up and allows the passeges to shrink.
You could also try using a Neti Pot every day or a saline wash in your nose to get things moving.
If you are having bad headaches, pressure over your cheekbones and eyes, and a thick discharge then you need to think toward an infection.