Is a Wifly shiled enough to achieve that?
Or pair of Xbees?
Yes both will work. Both are relatively straight forward to use.
A WiFly RN-XV on a supported XBee shield is a bit simpler to use, but you will need some software on the PC to receive and send the messages via the network. What's your favourite scripting language? What's your PC Operating System?
The XBee can behave like a serial interface, so with that respect it may be the simpler solution for you.
You may also want to consider cost.