Does the Squeezebox say it can't find the network or that it can't find SlimServer?

You haven't inadvertently turned off DHCP on the router have you? This would mean that the PC and SB are not being given IP addresses.

Are you using WEP? If so, does the SB have the right key?

Twin-aerial routers throw and good signal fore and aft but not much out to the sides. Consider this when positioning your router. This caused me problems when I first had an SB.