I would blame the wireless adapters way before the router. I currently have a Netgear WPN111 usb adapter on my desktop and it is only getting three bars and I'm upstairs in the room directly over the router. Literally less than 20 feet from the router. I had a Belkin usb adapter (I usually hate belkins stuff) and it will get five bars sitting at the same usb plug as the Netgear on my desktop. Not a thing changing other than the adapter in this case.
I have hooked literally thousands of wireless networks and seen the differences in how computers connect to the same router. Amazing the difference that the interanl card in notebooks make, and in some cases the same cards in different laptops work really different (I guess the internal antennas are vastly different) and drivers can make a big difference too. Normally the router is fine, check the adapters being used. Also, if possible always use a USB adapter as I find better signal on these over internal cards by far 95% of the time.
PS - the wpn111 is a horrible adapter, never get one lol. Overheats and locks up and makes me lag in games, the belkin works perfectly.