I’ve done a lot of reformatting of older computers, in order to keep em running as new, and one of the things i’ve found helps the most, is keeping the number of installed programs as low as possible. That, and not overfilling the hard drive in general (w/ music, videos, etc.), will by far be the best way to keep it running smoothly.
I would definitely put either XP, or maybe even Win2K on there if you have a copy of that. If you can get your hands on a stripped down version of XP (such as the Black Edition mentioned above, or something from a friend, maybe), i’ve found that to be a good help.
As for programs you should install..
* Firefox is good, but Chrome uses a lot less resources, if she’s into trying it out
* Anti-Virus and Anti-Spyware are essential. And as much as there are a bazillion free ones out there, I’m a personal fan of Norton (Symantec). I mean, you use your computer EVERY day of the year, quite possibly, which means that even if a subscription is around $60/year.. that’s only SIXTEEN CENTS per day to keep your computer safe and sound.
* Show her how to Defrag her hard drive every month or two (accessories, system tools, usually)
* Open Office is fine, but if you have a version of MS Office, you may as well not make her learn new programs.
Beyond that, if as you said, she doesn’t use much else.. then you should really just go into the Add/Remove Programs utility, and uninstall just about ANYthing that’s not a program you just installed (Internet, and MS Office, and your Anti-Virus), or that’s clearly a Windows program or Windows Update. Everything else should be uninstalled.
If you want, you could even go so far as to delete all of the “sample” items that often come included with Windows.
Good luck! and post another response if you have any problems come up.