I have a heat sink fan that is close to its end of life. I bought a replacement and attempted to open the case to replace the old fan but after removing all of the screws the case seems to be held along the back by some mysterious fastener. While I would love to replace this noisy fan, I don't want to destroy the computer doing so. Any advice on opening this model? It's a B 120 ...typo in the subject line. I do think the 120 and 130 have basically the same case.