It would knock off all users and some routers would reboot or lockup. But try the router's latest firmware and router defaults. Set a simple SSID and enable only WPA/WPA PSK security.
Outside of any ISP changes that's it.
HOWEVER the details are so slim in this post that's going to be my only thought other than try another router.
We have a Linksys WRT110 wireless router. It works fine with only 1 laptop on at a time. However, if a second laptop tries to get on, it can't. The second laptop can be on via the wired ethernet, no problem.
We have the same issue whether the laptops are running XP, Vista or 7, so it doesn't appear to be OS-related. Seems like a router issue.