So instead I installed Ubuntu (Intrepid) on an old machine (Athlon XP 2500+, 1GB RAM), set up my admin account and her limited account, and set up SSH on it so I could remotely administer it if I needed to. Total installation, updates and configuration took maybe 2 hours
So far it's been a month and I've never even had to go help her with...well, anything! She's been using it daily, and I've not had to fix anything.
I saved time from having to install antivirus/spyware-scans, installing updates and service packs (I only have a slipstreamed copy of SP2, I haven't had a reason to bother slipstreaming SP3 yet), it's nice and snappy even with just 1GB RAM (certainly helps to not have to run an on-access virus scanner!) and it literally does everything she needs.
Forget Windows. Seriously, what's the point for someone who just wants to go online? Letting the "average user" go online with Windows is walking into a high-school with a kick-me sign on their back.