If you're sure you have an HDTV, HD box, & have it connected via HDMI or Component video cables the most likely culprit is the settings on the box. Quite often the cable company recycles those units back out into the world without resetting them. The last person who had it may have set the resolution to 480p. Tune to a known HD broadcast (i.e. Sportscenter) and check the resolution again. If it is still 480p then it is probably the settings. Without knowing the provider or the model of the set top box I wouldn't be able to help you any further. Call your cable company.