Copy your family’s unique files (docs etc) onto a flash or portable drive, or cloud storage. Open computer. Remove your existing windows HDD and keep it safe. Fit an ssd drive in its place, 120G should be fine, they’re still cheap. Install your linux distro of choice. Copy over the docs from storage. Then, you know you have a full path back to where you started if you get shouted at. Just swap the drives over and you’re back in slow windows land. Yes, running linux in a vm works fine, as does dual-booting, although the latter runs a risk of making full recovery impossible if it goes wrong. A 10yo computer may have a mechanical hdd which will be slow whatever OS you use, but unless you’ve kept up with Windows updates, it’ll also be very unsafe to be using online. That may be the reason why it’s suddenly running slowly - it’s got malware. Installing a new OS would solve that too.