Not to difficult.
Just put a big icon in the middle of the screen lable it "Word" , have it launch abiword when clicked(a person used to word will have no problems with abiword). Firefox labled "browser" and thunderbird/evolution labeled "e-mail". and do the same for whatever other program they need.
What else do they use it for? If it's for business and such I can understand exactly sticking with Windows, most people need specific features that are only aviable easily in Windows.
Actually I would install Win98 for the same purpose, but you have to understand that Win9x support is mostly gone from MS. Patches and security fixes are going to be in short supply, and if your going to be the one dealing with it, what more do they have to know about how Linux works?
I just wanted to point that out.
Not that I think it's a big deal either way, you know your parents MUCH better then I do.
