As far as laptops go, you're stuck mostly. Dell is what I would recommend for a laptop.
Desktops...find your local brick and mortar PC store (one that hopefully has a good reputation) and let them build you one, or custom build one yourself for her. HP, Compaq, Sony, Dell, and pretty much all the OEM's use at least some proprietary parts. Typically these are cases/motherboards/power supplies. Sony used to use a special CD-ROM in their computers which was required to reload the recovery CD. Regular CD ROM's wouldn't read them. Yes they come with warranties, but a good brick and mortar store will offer a 3- year warranty option. The one I work for does. But a custom build is really the only way to go with a true reliable upgrade path. Also, if parts fail after warranty, replacement parts do not cost ridiculous amounts. I've seen Compaq charge 100-200 for a 200W power supply just because it wasn't standard and you had to get it from them.