Hi.
This happens all to often. I am a professional photographer so I need my files to be 100% safe.
I do use mac, but hard drives are mechanical things and they can fail on mac, linux or PC
I have all files on local hard drives, plus copies on a hard drive off site. backed up EVERY DAY in addition long term storage of projects over a year old are stored on a hard drive and on a DVD....
When it come to my private files I do the same. So far I havnt lost a file even after a lightning strike came down the telephone line and in and out of my mac frying everything!
Nothing is foolproof, but the more backup you have the less chance you have of losing files.
As you have installed new operating system on the drive you could well have written over some or all or none of your photos.
If you cannot find the files on the old computer then take the one that got the virus to an expert than can try and recover the files.. the more you use that pc now the more chance you have of writing over something.
Good Luck
Nathan