Modern Linux distributions like Ubuntu, PCLinuxOS and Mandriva are incredibly userfriendly. I've set my grandparents computer up with Ubuntu and they like it. Ofcourse all they do is surf the interwebs, read PDFs and mail, but still.
They haven't event noticed that they now run Evince instead of Acrobat Reader, OpenOffice instead of MS Office and Evolution instead of Outlook. All they noticed is that they're system just acts faster, and does less complaining.
I would not recommend though Mike, that you install Ubuntu on that box of yours instead of XP. My mother had XP running for years on her 400mhz celeron with 256mb RAM, and it ran smooth. Ubuntu cannot do this. Took me ages to get Ubuntu running on a 1400mhz Centrino, 248mb RAM laptop. And it still runs slow (Had to use Ubuntu on that laptop, company policy)
Ofcourse there are other distro's with a way smaller footprint, but this will be a poor solution, unless you are or know a linux geek.
IF you are the geeky, effective type though, i would recommend Plan 9.