I use openelec distro of XBMC on my Pi, you can map the Pi to your PC in My Computer and then browse your media. For example mines located at \\192.168.1.10\Media\Media Storage(Name of HDD attached to Pi)
Using VLC i can then open any file stored on the HDD attached to the Pi and play on my PC. I would recommend using a cabled connection as wireless is not good enough for streaming large media files 4-6GB upwards probably wont work.
You will have to turn on UPnP and allow devices to control the Pi, this can be found in XBMC settings
Hope this helps