I feel that Vista and XP are still fine to use with an up to date browser and a good anti-malware. I've had great results with Panda Free. I've put out several Vista units lately that ran beautifully.
That said, if you want a new unit, fine, but if not, converting the existing to Linux is another very viable option. I've converted several, and every one has been a success. In fact, my own unit, which I'm on now, is a Vista machine, upgraded to Win7, and now converted to Linux for the past half year. I don't plan on going back to Windows, especially with the turn MS has taken. There's no need to. And it's all free. The version of Linux I use, Linux Lite, runs faster than Windows, so older hardware gets a new lease on life. There is some user adaptation that must take place, but LL keeps it to a minimum.