>>(My TV has a HDMI connection which i've plugged the pc into)>>
That would appear to be the clue.
Your Humax digital TV receiver (presumably a Freeview set top box) converts the digital signals back to analogue for viewing on a standard TV.
It's the same for a computer PCI or USB TV card if used with an analogue monitor or TFT.
My Twinhan DVB-ter D+A PCI TV card is HD compatible but you can't get HD signals for it at present.