Hey, you're getting a few things mixed up. read what i put very carefully to see the differences in your idea of wireless jargon and the methods out there (not to making fun of you, but asking if yu can set your router to ad-hoc tells me you have no idea...)
yes, there is. I don't have my laptop in fornt of me, but IM me for my number (you live like 20 miles away) and i'll gladly walk you through it. ad-hoc menas you network without a router, ie, not infrasturcutre. its basically device to device, with no central hub.
Do you want to set up wireless file sharing period, or are you leaving the network and need to filehsare between devices connected to that network, or are you trying to connect to a single device and share files through that? To share files through your network, both in adhoc and infrasturcture, you have to enable shared folders in your "security and sharing" alt click or in properties on the harddrive/folder. You cannot share individual files, only folders, so you'll have to keep acopy of the files in the Shared Folder, whichever folder you decided it be. Then you have to make sure that both computers are in the same workgroup. Then you find the the files by mapping it as a drive. Another wayis Remote desktop, but we'll get to that later.
You cannot have an ad hoc connection with a computer in infrastructure mode. Why? becuase you get one setting per network device. IF you have two networking cards, then you can bridge the connections with the wifi being adhoc, and the infrastructure/ehternet connection being hooked up to the general network, and bridge the two connections. that's all in network properites.