I will try to develop and detail the issue as much as I can.

I am connecting to a wireless network without any security (via DHCP). All is fine as the DHCP functions. When it is down (due to some switch hanging upstream) it self assigns an IP and not connect to it. With my previous PC (HP Compaq) I used to enter a manual IP and DNS and it would work...on my MBP...it won't even with manual IP (although my colleague who has the same MBP type could nevertheless connect, as well as the Windows fellows).

Now...is it something physically wrong with my lan card, or my system (after reinstall same issue) or...it exceeds my knowledge.

I will be grateful for any good answer.