Being able to share the files is a good sign...it eliminates problems with the files, your network settings, your motherboard, and probably the HD itself.
However not being able to boot from a CD and incomplete booting are bad ones....this could mean your optical drive is failing and/or has a bad/loose connection, and a borked OS....
a failing battery could be a link in the chain, since it maintains the pram settings. Have tried to reset the pram (cmd+option+p+r keys) ?
my other guess would be the corruption I mentioned earlier, it very well could be a combination of these things. Keep trying, the answer will become evident sooner or later
