There's most likely some outside signal interference. Your wireless devices will adjust the data transfer rate to compensate.
Check for a 2.4ghz cordless phone in range. Roughly 50 to 100 feet.
Microwave ovens will throw out 2.4ghz signals.
Some wireless security cameras will blanket a wireless network effectively drowning out the network signal. Range will vary but again roughly 100 to 200 feet.
Other systems on overlapping channels will cause a harmonic distortion of the signal degrading it.
Ok so that's what you could have going. Simple test for other systems on your channel is to move the wireless router to another channel. Then click the icon in the tray and let windows show you if it sees it. If it does then click on that and check the speeds.
But just so you know it may say it has a 108 or 54 mbs connection but the data transfer rate is a different story. Way too many variables.
In the router main set up you have a channel selection. Make sure you hit the save button. Your computers "sniff" the airwaves looking for wireless networks. Open the network icon and select your network.