My computer is very slow for watching TV and video's - the verbal is ok & it used to be fine - what can I do?

Until recently I could watch Catch Up/On Demand TV quite well on my computer. I am not sure why but I can't seem to access anything at the moment - I can hear the verbal but can't watch anything - it either freezes or does not play at all - any suggestions - I am rather a computer illiterate.