Need advice - can't connect to the internet on my macbook!

Hi

I have a huge problem! I can't connect to the internet using my macbook, however I can connect to it via my Imac and my PC laptop.

I have full bars on my Airport network but when I try to access a webpage on safari it comes up with the error code that my computer isn't connected to the internet. I then go through network diagnostics, and go through all the stages until it asks me for my wep password, I put this in (which I have double checked with my keychain program) and then it comes up with 'Network Change Detected' - your network configuration has changed. Click OK to proceed to the next step. Click OK and then all the bars disappear and it comes up with an error message that I was unable to join Airport Network. I have a sky router which, as I said previously seems to be working fine as I can access the internet through my other two computers. I can connect to the internet via my Macbook with ethernet but as soon as I take it out I get the same issue. As far as the network status goes, I have a green light for Airport, Airport settings, an amber light for network settings and a red light for ISP, Internet and Server. When looking at my system preferences it tells me that 'Airport has a self assigned IP address and may not be able to connect to the internet'

I would be really grateful for any support as I am totally stuck.

Thanks in advance,

Jules