Why cant i get hd tv?

I have an hd ready tv and a virgin plus box but when i try to access hd programmes it tells me that my tv isnt an hd one. also, while were on the subject, why are the pics from all my cable channels through virgin so much worse than the pics on my terrestrial channels through my aerial. is this whole digital thing a complete con?
Answer
I am assuming virgin is a cable provider and the virgin plus box is a cable box you are using. if the "plus box" is a cable box then a tuner is not necessary. with that, it could be two things. either the cable you are receiving is not digital cable, which is necessary to get any hd programming and would make it look horrible, or you are using the incorrect cables to connect to your tv which would also make it look pretty bad.