You can emulate a usb keyboard with the ATmega328 using V-USB : http://www.obdev.at/products/vusb/index.html
If the computer has a serial port or parallel port or PS/2 port or game port, you could use that.
Or using bluetooth, with a bluetooth dongle in the PC.
Perhaps using ethernet or wifi.
And beside those, you could open a device (like a keyboard or a usb audio stick) and connect your ATmega328 to the circuit board of that device.