A couple things you might check.... Is it only the wireless that disconnects? If you have a hardwired computer does that one stay online?
If so, it may just be a wireless issue caused by some sort of EMI.. a couple things you wanna make sure of... Make sure your wireless router isnt anywhere near anything that's gonna cause EMI... computer monitors, TVs, microwaves, Refridgeorator, ect..
also make sure that your wireless router isnt gonna fight for wireless control of anything else that might use a 2.4GhZ connection... walkie talkies, baby monitors, cordless phones ect..
after that if you still have the problems I would sugguest moving the router into another part of the house and see if that lowers the occurances you see.
If the hardwired computer gets disconnected also check and see if the router is rebooting. if so I would make sure the router isnt connected to anything that would hinder the power... Extension cords surge protectors ect... try it directly into the wall outlet. hope that helps