Weird problem with my network - It's meant to be infrastructure only...

Hi,

Before I went on holiday, everything was working fine. I came back yesterday, turned on my Mac, and connected to my wireless router. But I have an odd problem which never happened before - now when I type in my SSID and the password in the "Other" section of Airport connections, it connects to my network, but the network's name appears under "Computer to Computer Networks"!! So I went onto my laptop (M$ XP Pro) to check it out and it showed up with my apparently "AD-HOC" network which was unsecured! AND I know for a fact that my SSID broadcasting is disabled, so I now have a hole in my network because of the Mac. Please may any Mac gurus out there help me? All I want to do is connect to my router via Infrastructure again....Cheers,

J_K9