Here's something I've been putting together that will accomplish this. It went on hold while I bought a Sparkle Power SPI220LE to power the PC, but I've found that it's standby power consumption is actually worse than my old power supply, so this is back on the boil:
Basically, I use a small 10W 5V switcher to supply the 5VSB power. The enable line from the motherboard to the existing power supply gets cut, and this gets inserted in the way. When the PC is in standby, the main power supply is shut off via a relay in its AC line connection.
I intend on connecting a couple of IEC outlets to the switched line, so that my audio amp and monitor get switched off when the PC is in shutdown.
The data for the switcher doesn't give idle power, but it's supposed to be 75% efficient, so at worst I figure it'll dissipate 2.5W at idle.
The whole lot goes on a PCB that's 1" wide by 3.5" long:
But it could just as easily be built with Veroboard or similar.
In my application, I have the PC (12W standby), my audio power amp (10W standby), and monitor (10W standby). Hopefully this'll reduce that 32W to 2ish.