Forget vista - it's going to be a nightmare. WinXP has been patched up -why go out of your way to experience a full new set of bugs?
I would stay away from Dell - I've had a very unpleasant time with their tech support, and you don't want to make your mother have to put up with being on hold for hours at a time - she'll likely make you do it instead. My first pics would be the Apple MacBook or any Thinkpad - a refurbished Thinkpad is often an overlooked bargain, as you get both excellent build quality (it's a Thinkpad) and a long, top-notch warranty at a very reasonable price.
If you want an "uber-1337" computer, I'd go for a HP/compaq instead, or a MacBook Pro if you feel like dishing out the cash.