Network Conundrum

Hello all - first let me say I'm not an IT person, I just got put in the role in the office as I know some about computers so I will do my best to answer questions and give mad respect to all in the profession!

I have added a new computer (windows 7 professional) to our network (windows 2003) which has 9 computers. This computer (new profile I'll call LD) replaced an old computer (old employee). I am able to connect to the network and see all of the files, etc. However I am unable to connect to the internet via LAN. If I turn on the wifi I can connect to the inter net that way. Now here is were it gets even odder - if I turn off computer AB I can get computer LD to connect or I can connect AB and LD cannot connect - this is to internet only (network is still connected).

Help please!!!!