sharing files of wifi will be very slow as your network is not optimized for this sort of load, what you want is a cat5 Ethernet cable which you should have one nocking about they come with most router or pc's.
plug the cable into both Ethernet ports on each pc, then click on my computer and paste this into the address bar
"Control Panel\Network and Internet\Network and Sharing Center" without quotes this will take you to the sharing center,
then go to the pc with the files you want to back up, open my computer, right click on the disk drive you want to back up or even the individual folders and click property's, then click the sharing tab, proceed to click the advance sharing tab, click share this folder then apply and ok. now go back to the computer to receive the files you should be able to see active networks somewhere with a logo of a house a bench or infrastructure click this and you should be able to see your other PC if you click into the pc you should be able to copy files over at a rate of around 56mbps (the limit of your hard-drive) if you was to transfer by wifi expect 2.5-3.5 mbps.
if you come into any problems let me know, and sorry for the poor formatting this was just a quick write up of my knowledge