Mac Forums

I have set up my MAC to share my USB wireless broadband data card with my other computers in my house without a problem. The only issue is that the wireless connection is open and unencrypted. I tried to setup a passcode and encryption using WEP (both 64 and 128 bit) and followed the instructions exactly given by MAC, but my other computers which are both running Microsoft Windows Vista keeps saying that my network and its settings are not compatible and the computers will not connect to the network. I sorry for such a long question but this is the first MAC that I have owned and I wanted to cover all the bases. I really appreciate your help.
P.S. I bought the computer on ebay and the warranty has expired thus I am not eligible for Apple Support.