Make sure your router is broadcasting the SSID and make sure it is operating in mixed B+G or B-only. If the router has enhanced/extended G features (advertised as working at 108Mbps), disable this feature. In addition to disabling encryption security (such as WEP and WPA), also make sure MAC filtering is off.
Of course, also make sure the PPC WiFi is turned on (I know this sounds silly but I've had this be the problem with a couple of people)
If you have trouble figuring any of this out, post the model number for your equipment and I'll check the manual.