I maintain that your best bet is to put Ubuntu Linux (very user friendly and easy to install - easier than installing Windows) on that old Pentium machine, and put that on its own VLAN. She can use your existing monitor that she likes if you buy a cheap KVM switch.

You can install a fully functioning Ubuntu VM in Windows to check it out - go to ubuntu.com and download the installer. It shows up in Windows as just another program that you can uninstall in the Programs control panel. Ubuntu comes with Firefox (Chrome is also available) and a fully functioning LibreOffice suite that is compatible with MS Office. It also comes with games, media programs, and the like. Don't worry about DRM and such - install Ubuntu and see for yourself if it works.

If you really like the idea of a Windows VM, go for it. But I think it can be a bit of a hassle to set up.

Also, I'd suggest she start using Chrome instead of Firefox. Chrome is generally considered the most secure browser.