If the kit is reasonably close, and your PC has audio out and your hi-fi audio in, you could do it via audio phono cables. There are also esoteric boxes which can log in to a wireless network but you're talking a fair amount of money and complex setting up for mixed results.
I don't think Belkin do it now but I got their Tunecast FM transmitter. It's aimed at iPods to transmit the output to a car radio but works just as well in the house. With a piggy back adapter from Maplins you plug it and your speakers in to your PC sound card. Then whatever sounds your PC is playing can transmit via FM to your hi-fi - always assuming it has a tuner section. The results are pretty good and total cost around £25.
If you have an FM tuner I'd google for FM transmitters/senders and see what comes up.
It's about the easiest and cheapest way if you can't physically connect the kit.