Vista - cannot connect wirelessly

We have a wired/wireless network with one desktop (wired) and three laptops
which ran wireless. Two laptops were XP and one was 2000. Just replaced one
XP and the 2000 with Compaq Presario running Vista Premium. Problem is that
I cannot make a wireless connection. If I plug in an Ethernet cable, the
laptop connects to the internet, but it won't connect wireless. The error
message is low signal strength or the modem is not responding. I have no
idea how to fix this. I have downloaded the Fix for XP and have disabled
firewalls. Any other suggestions? I'm not worried about sharing files at
this point, just want to get the laptops on the internet for now.

Thanks in advance,