I do know that you were able to find your installer DVD, so you will likely need to replace the hard drive, as it sounds like it is simply not working good enough now.
The hard drive is not difficult to replace in an older MacBook, but you may be more comfortable in having the job done at an Apple service shop, or at an Apple retail store...
The reason that I say that is that I know you have some question about the fan working properly, and a shop can check that fan out to make sure it's working OK (and actually cooling your MacBook when needed)