I cannot connect to my wireless router with wireless card on pc

Hello,
i have a router which i have been using for some time now with 2 computers, one is wired and it uses XP and the other is a laptop using wireless and it uses windows 7. now i have bought a new computer that also uses windows 7 and installed a wireless card on it.

the signal from my router sometimes comes and leaves after a while and sometimes is not visible at all. whilst from the same place, the connection from the laptop is full meaning that the range of the router reaches this place very well.

when my neighbour's router is on i can connect to the internet through it sometimes but it is also a very poor signal.

the strange thing is that i tried the wireless network card in another pc and the connection to my router from the same place is full. therefore the card is not the problem. i also tried another card and the same thing happened.

if someone met this problem before or has an idea of what i can do, please let me know