By Original Poster: The prob is once it's started and when I try to do anything, the window I open dims and then freezes or the whole computer freezes. Sometimes I can shut the frozen window down, and then open another application, i.e. Firefox, but I can't do anything in it.
The screen dimming part really sounds like a hung process. This could also cause a the lack of response. Using a terminal, what is the output of the command top?
While I agree that an older laptop will have a degraded performance compared to a new(er) one, this seems a little extreme. If it is possible I would also try to verify the ISO image and the install CD md5sums to be sure that you don't have a corrupted file or driver causing you problems.