Is it possible to retrieve photos and files once a new windows has been installed?

[SAH]

so my brother accidentally installed an anti virus pop up that said our computer was at risk an it was recommended to protect it. so then the thing installed and it scanned and found like 22 viruses and some were critical like trojan and stuff but anyways so that happened and then the next day we see that some sort of virus was eating away at our photos and just deleting them and then we had to install a new windows vista with all the other files gone so i want to know how this happened and if there is a way to get those back