i am trying to set up a wireless network to our guest house, where my son lives, but the signal from the main house is too weak, i have an extra linksys wrt54g, is there any way to use it as an acess point or repeater in the guest house?
It can be done. There is also an old thread around OSNN by someone who did what you are asking about. If we can't find the old thread here Google "setting up a WRT54G as an Access Point".
Ideally the repeater should to be midway between the two houses. to extend range. You could place it at the guest house to boost the signal throughout the house but you are still limited by the quality of the signal it gets from the main house.
Place the main house router as close as physically possible to the guest house, by a window will help unless the window has aluminum mini blinds.
You could also put range boosting antennas on either or both routers.
Cheapest improvement would be to put a homemade windsurfer antenna (google it) on the router in the main house aimed at the guest house. It makes the signal directional and boosts strength by a factor of 2. Direcitonal may cause issues at the main house. To avoid messing up your main house coverage, set up the other router at the guest house with a windsurfer aimed at the main house.