Try not to worry man,WIndows has a habbit of making Wireless networks dissapear after a failed attempt making things a bit confusing (well for me atleast!! )

ok, jsut to recap, all security is off, and the "Create Closed Network" box is UNchecked (under Airport admin utility) make sure that you update after changing settings other wise the setting wont stick.
An update should take baout 45 seconds to 2 min

How To Make PC Finds AirPort Network
 

Hi Jonski,
It seems I know the problem. I've encountered the same problem then I finally find out that Most PC can't find AN AUTOMATIC CHANNEL setting from AirPort. Set the channel to your favourable (1 to 12) and try again. Enjoy.

http://adinoto.org

I had problems getting my partners windows box to find our airport network..... her daft laptop has a button on the top to turn the bloody wifi on and off - and it was off! :O

Solution
 

Simple answer: change the channel!

I have a set up with one mac laptop (which can always see the network) and one windows laptop (which recently mysteriously couldn't see the network), logged into the airport with the mac and changed the channel setting from "automatic" to a lower down channel et voilà the network works for the windows laptop.

Hope this is of some help.