Some People not connecting?! Wireless network problems

I have just set up the wireless network for my house at uni and we're having some problems here. I can use the internet fine, another housemate is also using the network fine but a third member of our house is having problems.
No idea what's going on but she is able to connect to messenger and skype but when trying to use firefox it won't connect to any websites. She is also being shown that she is connected at 24.0 mbps while myself and the other person get 54.0 mbps.
There are another 3 housemates to connect to the network but they aren't here at the moment so I can't see if they are having any problems. The two laptops that are connecting are the oldest and the newest laptops in the house so I don't think it's a hardware problem. Add to this the fact that the router was used in our house last year and she was able to use it fine then.
It's really annoying me now as I'm the one trying to sort all this out so any help would be apprieciated.

 

Forgot to mention the router is Netgear WGT624

 

I've seen post with people having problems with those routers,but it doesn't sound like its that.Try updating the wireless card drivers and see if that helps.

 

If she's using Norton it may be blocking FireFox and not learning it as an exeption.

I had that happen on my laptop.

If she has Norton 2006, have her play around with it (disabling it for a few minutes, etc) and see if that helps.