My macbook pro (OS X 10.8.2.) can no longer connect to my wireless

Please can anybody help me I am at my last straw ...

My macbook pro (ML OS X 10.8.2.) will no longer connect to internet, it somehow does not get the IP from the router (Belkin) and the DHCP will not renew.

The problem is that all my devices and other laptops in the apartment work absolutely fine, and my macbook worked also for about three months, then all of a sudden I woke up, opened my mac and no internet anymore ... it works if I plug it directly tot he modem, so the internet is fine. I also tried changing three routers already, always the same problem.

Is it possible that the issue is the new ML softwear? ... I really have no more clue what to do ... I called my internet provider many times and they have no clue, I also went to an apple stre and they said its my firewall, which I turned off as they suggested and still not working ... funny thing though it worked in their store, the wireless was flawless, so I came off like a retard ... it also works at my bro's place who has linksys router ... so I tried buying Linksys E900 but I cant even install it now because my mac does not detect it ... it will connect to wireless so the airport bars are full, there is just no internet coming through ... its like my router is preventing my mac from connecting ... also checked rtouter settings and googled a lot, and all the settings should be fine for mac ... I dont get it

I simply have no more solutions or options I can think off ... so I am quite frustrated about buying Muntain Lion at all ... I never had such problems ...

plese if anyone can help I will be immensely grateful, I spent so many days and hours trying to figure this out, but I am no computer expert so I have absolutely no clue what else to do ...