What ISP Service do you have? Cable or DSL?
What ISP Modem do you have? Stand Alone or built in router?
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual?
Turn off Advanced DNS Services if you have this option.
Turn on DNS Relay under Setup/Networking.
Setup DHCP reserved IP addresses for all devices on the router.
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP.
Turn off all anti virus and firewall programs on PC while testing.
Turn off all devices accept for one wired PC while testing.
What wireless modes are you using?
Try single mode G or mixed G and N?
What security mode are you using? WEP, WPA or WPA2?