DNS problems with Snow Leopard. Can't upgrade to Mountain Lion either.

Question

My Macbook Pro has been working quite well since I got it just over a year ago, until last week when I shut it down to go stay at my parents house. I turned it on when I got there and it automatically connected to the network because i go there often, however when I tried to open up a browser it would not work. I opened up several of the other browsers that I have but all of them kept saying they couldn’t connect to the server. I thought perhaps my dad had changed the router settings or something since last time I'd been there since the internet was working perfectly at my house only two hours previous. He said he hadn't changed anything and he was right, the internet worked perfectly through my sisters PC and through his ipad and my iphone. I later took it round to a friend’s house where it remembered his network too but still would not open addresses in the browser and when I finally got home to my house, I found I had the same problem. With every network I connected to I could update my apps and software, use my torrent client, ping certain IP's and connect with various other things that require internet connection, but the browser still never worked. After some research I came to the conclusion that it was most likely a DNS issue, I found this odd as I had not changed any sort of settings at all recently, except for turning the firewall on, which I turned off be3fore trying to troubleshoot. I  have tried virtually everything that has been suggested on the internet like Flushing DNS cache, resetting PRAM, bringing my network to the top, turning airport on and off, connecting through ethernet, renewing DHCP lease, using google IP as DNS, checking for set proxies, repairing disk permissions, resetting router, checking router settings, the list goes on and on. The last thing I tried was to copy the com.apple.mDNSResponder.plist file then delete the old one and paste the new one back in, since then my Mac cannot connect to the internet in any of the previous ways.
After pulling my hair out for days, I thought the only option left was to upgrade to Mountain Lion, which I was planning on doing anyway. After downloading it though, I came to the option to select the disk I want to install to only to see my primary and only disk open my Mac coloured faintly and telling me that "This disk cannot be used to start up the computer". After more research I found that there were a few issues, none of which I have. I’ve never used time machine so there is no issue with any set partition for it, I have no other partitions on it at all either so I cannot use the resizing trick that seems to help most people. I would normally just backup all my data at this point and do a fresh install, but haven't got the funds to purchase a another TB at this point and I have over 650GB worth of valuable data on the Mac. Could someone pleeeease help me!?!?!

Answer 1

You cannot go to Mountain Lion, as stedman1 states.
But you can upgrade to Lion, possibly.  See http://support.apple.com/kb/HT2404