Simply connect the speaker output from your pc to an input on your hifi, you'll need a stereo 3.5mm jack to twin phono lead.
Some pc's are not good at minimising mains hum, so if you get buzzing you'll need an inline surpressor.
However depsite having a PC connected to my system, I use the BBC iPlayer on the Wii more often that not.
It came in handy today when I realised SKY+ didn't record the full grandprix when BBC swapped it to BBC2 half way through!!!!