What OS is she using? For any modern OS, I would upgrade the RAM before trying to install new software. 256 MB was too tiny for any serious work with Windows XP or later. If she is running Windows 98SE or Windows Millenium I would not bother to upgrade the RAM, Reader or the version of Windows. Its would be time for a new machine.
Thanks for the reply. She's running XP. The computer is about 5 yrs old. I suggested that she get more memory, at least 1-2GB
Version 9.x Reader will uninstall versions 6-8 before it installs 9 automatically without any intervention on your part. Thankfully Adobe "wised up" after version 5. Version 5 or older require manual uninstall prior to installing the latest Reader.
Good luck with the RAM upgrade - make sure to find out the maximum amount and type of RAM the system can handle from the manufacturer!
Hope this helps.