How truly safe is VirtualBox with an old computer?

Please forgive me because I know this sounds like an incredibly stupid question for newbs. I wanted to install VirtualBox on an old computer and then have the computer connected to my router so it can get access to the internet. Although it will be connected to my network, no other PC in my house will connect to it.

I'm going to use this computer to watch stuff like PubFilm sometimes, go to websites that I don't really trust, download 3rd party programs in order to test them for a week or two, etc. I would put this virtual machine to heavy use.

My question is this:

How likely will a virus be able to break free through VirtualBox and spread throughout my network to my family's PC's?

I actually looked up the answer already on the internet and some said that VirtualBox actually isn't that foolproof and there is a good chance! Some say otherwise of course. There seemed to be mixed answers but generally people safely say that it's not 100% safe but it's pretty decent. But I'm wondering how likely percentage wise will the other PC's on my home network will get infected with viruses due to this virtual machine, within say a year's time?

I'm asking annually because I'm thinking maybe the safe thing to do is have everyone in my family just re-install their Windows fresh every year to make sure they minimize the damage of all possible viruses.

:)