Assuming you don't simply want to crank up the volume of your PC speakers, you're going to have to buy a wireless music streamer. Once you've connected the audio-out ports on the streamer to the audio-in ports on your hi-fi, it plays the songs from your PC over your home Wi-Fi network.
To use a streamer, you'll need to have a Wi-Fi router set up already and install the software that comes with the streamer on your PC.
There are a few models to choose from, but which one is best depends on the format you've ripped your CDs in. Well done if you've chosen to rip CDs in the MP3 format, because this guarantees your music will play on any of the models available today.
If you've used Microsoft's WMA format, however, you need to be more careful. Most models support the unprotected version of this format, but not all support the copy-protected flavour. If you've bought or rented songs from services like Napster, you'll need to make sure your music streamer can cope with copy-protected files.
Those with iTunes will have ended up with music files in Apple's AAC format, unless they have changed the default settings, so check the streamer supports it. If you've bought music from the iTunes music store, you'll have to use Apple's Airport Express with Airtunes. This is a basic music streamer that, unlike rivals, has no screen, and it's really only practical to control it using your computer rather than running it separately using a remote control.
The model we'd recommend for most people is the Slim Devices Squeezebox 3. It's easy to set up and offers the greatest number of features through the excellent Slim Server software. This is an open-source program, which means that anyone can write a plug-in for it, such as the excellent AlienBBC, which lets you listen to BBC radio stations, as well as 'listen again' content. The only drawbacks are its high price and lack of support for copy-protected WMA files.
A cheaper option is the Roku Soundbridge, which does support copy-protected WMAs, but the PC software is a lot less sophisticated.
Audiophiles can turn to the Sonos Digital Music System, which offers the best sound quality of all the units here, thanks to its decent built-in amp, and a natty remote control. Opt for the Sonos ZP80 if you already have a decent hi-fi, as you can use your existing posh amp.