Well, mac filtering is a painful way of achieving a useless affect. I tried to use it to allow only the computers I know onto the internet (the purpose), but every time, it just booted them all off the network. From what I've read, it's pretty useless. Doing things like mac filtering, not broadcasting SSID, etc...only serve to make it more difficult for you to connect to the internet. The signal is transmitted in many more ways than that. Somebody with the right tools can still find your network even if you do things like that (mac filter, hide SSID, etc.) The best way to stay secure is to use WPA2 encryption with a long password (random, non-dictionary based ASCII characters, like !*?R#PZPwv^
Sounds like that email may have infected your PC somehow. You can either hack in out in the "Am I Infected? What do I do?"
sub-forum (NOT in this topic!!), or you can do the simple thing and just reset to factory settings (my recommendation).
But yeah, forget about the mac filtering. Just use a strong WPA2 password, and reset to factory settings to get rid of any malware you might have picked up. If you had had it longer, I would say to try and fight it. But with one fresh from the factory, you might as well just reset it to factory settings.