i know the OP posited windows as a requirement,
but there's also a hybrid approach.
my primary goal was to protect my mom from serious financial fraud - ie bank account stuff.
mom uses windows for photoshop & other SW that won't go on a tablet,
and also makes heavy use of some windows-only apps which keep her away from OSX.
so, i got her a tiny HP laptop and put Linux (mint) on it,
with the strict instructions that all banking is done on the Linux machine,
and *only* banking is done on the Linux machine. no shopping, no surfing.
shopping i figure is fine on the windows machine: the credit card is secured against fraud,
and shopping itself is risk enough that it doesn't belong on the banking machine.
I suspect she's not actually following this advice, because she hasn't asked for help w/ the Linux box,
but that's probably also a failing on my part for not following up.