Help! wireless problem

Hi
I've had some problem with wireless internet, and am hoping to get some advice.

I bought my macbook pro in early April in Boston.
I accessed to the internet through a closed network there.

Then I came to NewYork and stayed in my friend's home for a while.
There were two friends, one used pc and the other used G4, and both of them connected to an open network whithout any ID of passcode. My macbook pro did it too, at first. However, from the second or third day, the speed became extremely slow, or it didn't catch any signal at all, or, even if it did, the network required a wep key so I wasn't able to access, while my friends' computers were totally able to connect to the same network.

After that I moved to another house, used cable, and everything was fine.

Now, I've moved in again, and have the same problem as that in my friend's house. I just can't understand why my macbook pro can't connect to the same network network while others can. Would it be anything about my airport card? I tested the hardware, but everything was ok. My firewall is of course turned off.

If anybody know the answer, please help me. I'llvery appreciate for it.
Thank you for reading this long thread!

 

Thanks!

Thank you for replying.
Anyway it's ok now though I don't know why.