How do i run my desktop PC from my daughters wireless router?

my daughter has desktop PC upstairs with a wireless router, my son runs his laptop from this and i have just got another desktop (not brand new) and want to run this from the router as my son does.I ahve bought a Belkin wireles USB network adaptor and installed it but i cannot get an internet connection even though the signal is strong. Icon at the bottom left of screen tells me 'limited or no connectivity' I don't know where i've gone wrong or why i cannot get an internet connection. I would really like some help please. I'm not really PC literate, so please be kind!!!