Before buying a wireless repeater and potentially spending money that you don't need to, download inSSIDer for your laptop:
http://www.metageek.net/products/inssider/
This will show you all wireless networks in range, and which channels they are operating on. Get in range of your wireless network, and compare channels. If there are multiple networks on the same channel, then log into your wireless router and change the channel it works on. Best settings are channels 1,6 and 13. This can help reduce interference and improve your router's wireless signal strength.
Also try repositioning the router. Electromagnetic interference will cause the signal strength to degrade just like a radio. Try to position it away from other strong electrical sources such as the monitor, speakers and base unit of your desktop, and away from things like cordless phone bases and the like.