What Hardware version is your router? Look at sticker under router.
What Firmware version is currently loaded? Found on routers web page under status.
What wireless modes are you using? Under Setup/Wireless/Manual.
Try single mode G or mixed G and N?
What security mode are you using? Preferred security is WPA-Personal. WPA2/Auto TPIK and AES. Some WiFi adapters don't support AES, so you might want to try TPIK only.
What wireless devices do you have connected?
Any cordless house phones?
Any other WiFi routers in the area? Use
InSSIDer to find out. Depending on if you have other WiFi around and if there are any open or unused channels, set the Auto Scan to a manual open channel, try 1 or 11 to test.
Turn off Short GI and Extra Wireless Protection if you have it. Under Advanced/Advanced Wireless.
Turn off all anti virus and firewall programs on PC while testing. 3rd party firewalls are not generally needed when using routers as they are effective on blocking malicious inbound traffic.
Turn off all devices accept for one wired PC while testing.