PC + Macs on Same WLAN Network Have Relationship Issues?

Right I'm on this again....

1. I reset the modem downstairs and now have admin access to it. Its the standard BeThere Speedtouch job which i find really badly laid out admin wise. I couldnt find a way to switch off DHCP on it.
   
2. There is someone who connects to that modem with an ethernet cable, so if I did switch off the DHCP on that and left it to the router upstairs the cabled guy couldnt connect, right?
   
3. There is a new PC in the house, and this gets perpetually stuck on "assigning IP", I can get into the router and modem admin pages but cant get onto the net at all from it. So I guess that reinforces what you guys were saying.
   
   

The channel isn't default, but i'll change it anyway....

Edit// ahh the router is setup by BEthere in there special way so you cant tinker with lots of things + the password and user name for the ISP is hidden in there somewhere so would have to get the logon details off BE and find a way to wipe the modem clean and then start again. In that case its too much effort ;)