A USB activated power board as per Space Fractal suggestion would be the ideal solution, however I haven't seen any where I live so I made my own. Cheaper possibly, but a bit of work.
I use a power board with the PC plugged into the first socket, with all the other sockets isolated. Powering on the unit is via the PC switch (obviously relocated) which via a free HD lead powers on a relay which then connects all the other sockets on the power board to mains. Pretty inexpensive solution, cheap 4 port power board (Sub, amplifier, PC, touch screen) + relay = $17. Press the button and the PC and everything powers up, to shutdown press the button again and everything turns off once the PC has completely shutdown.