try connecting a hard line to the router and dial into it. (192.168.1.1) put in the user name and password (may be on the bottom)
if not most old ones are admin and word password. try turning off wep if it has it on and check the speed the router is set for. some old routers can be b routers...slow then the newer ones are b/g.. make sure it set to g and 54 for the speed.
newer hardware is n. also if you have any 2.4 phones or if there a lot of other networks in your area try changing the default wifi channel. one last thing is could be a firmware issue..with newer chipset that are out there. when your logged int othe router check it firmware with ones online..if it old i would update it.