One of my favorite OSs for lame machines is Puppy. It is small, small, small and will run on 32meg of RAM like a Zebra with a Lion 6" off it's rump. I also specialize in old folks and being only 68, this month, I, just, cannot understand why I appeal to them so much. (The grumpy old coot pulled his oldest Fedora with the three inch brim down to the top of his glasses, turned, walked a couple of steps before giggling.)
Go Linux for your OS: The new Fedora 17 KDE seems nice-getting better reviews than Ubuntu, does have some initial update quirks. If you Like a windows OS, you might want to toy with an Ubuntu based Linux called Zorin. Just my opinion- there are lots of FREE Linux OS's to try.
I agree with building your own PC, much cheaper and better if you pay attention to the specs. I just built a dual-boot desktop system to run AutoCad for less than $600 that will cremate anything stock within the same price range.
You can buy just about any Laptop built within the last 2 years, throw Linux on it and it will probably smoke Windows.
I wont touch fedora, last time I used it, it seriously screwed up my partition table and made me reformat my entire computer. Granted, I was using the beta version, but its turned me off to fedora. My dad used to be a redhat beta tester and he said he liked it, but i just cat get myself to try it again. I prefer OpenSuse or Mandriva.