Buy a network signal tester and plug it into each port....

Nope your friend is wrong or you mis-understood them. No damage is done but there is an adaptive feature of wireless network equipment that can vary signal strength based on environmental conditions....

You need to configure Port Isolation.
http://www.h3c.com/portal/Products___Solutions/Technology/LAN/Technology_Introduction/200701/195584_57_0.htm
That website should help you get the basics.
I...

No it doesn't make sense but I was the only kid in class who managed to prove that in maths any outcome is possible.

Also I cheated and used this handy calculator....

No u can set a static I.P. as long as you set the TCP/IP settings on the laptop to stick to that address, in fact in practice u might fine that u only have to enter a manual I.P address on the...

Mac address filtering can cause exactly wot your describing. you need to go into the router's config utility and turn mac filtering off or add your daughters card to the allow list.

very tempremental..but try this...on both machines right click on my computer and select manage, click on services and applications>double click on services & select the computer browser service....

is the modem a router?

So let me get this right. Your new computer used to communicate with your old computer that is until you got the new computer? Did you ping it while it was a display model? lol

No sorry seriously...