Netgear MAC Authentication Question?

Everything was working fine - then this afternoon lost internet and dont know why...I have a HP desktop with XP, Netgear WNDR3300, lost wifi content, but still able to connect to network. Comcast ISP said modem is fine, and when going desktop >ethernet>modem, I can ping and they can see me. I set up the router again, and tried pinging from desktop out ethernet to router to modem and they couldnt see me. I logged into routerlogin.com and after an hour, found that if i switch the MAC Authentication under the "Router MAC Address" from my desktop MAC address to the MAC address of my Iphone - I get internet and again all the wifi works and is very fast on all devices. If I go back to desktop MAC ID, I lose it again. I tried the default option, and the manual one - Is my desktop somehow not being recognized as legit MAC ID? Sorry for the rambling, and appreciate any help!