Hooking up wireless internet on Vista machine...

I bought a HP Slimline computer from a local shop on boxing day last year with windows Vista pre-installed. I recently got hooked up to wireless internet, and for some reason, vista is unable to see it properly. It states there is "limited connectivity". I can use my wireless internet on another computer by simply unplugging the cat 5 cable from the network adaptor at the back and plugging into the other machine which has ME on it. The highspeed internet works great on that machine !!!
I am not using a router. The dish is simply bracketed to my tv antenna. An outside wire comes off the dish into my house, then plugs into a surge protector box. There is another cable that comes out of this and into my onboard ethernet card. This cable is also powered by an adaptor that simply pugs into a 120V outlet. It is a simple hookup, but Vista won't take. The installer said they usually hook up to a machine with XP on it and never have any trouble.
Would any one out there have any ideas on how to proceed ? Thanks in advance.