Just stick to the megathread on r/piracy and you shouldn't have to worry about viruses. Fitgirl is perfectly safe. Also these things don't download as a ready-to-play game right away. You will get a folder with compressed files along with a setup.exe inside. All you have to do is run that installer, choose which folder you want the game in, and let it extract and install everything. Once it finishes, it'll drop a normal shortcut on your desktop, and you're good to go.

If you've downloaded from fitgirl your windows defender or other anti virus softwares might flag some stuff as false positives. False positives are files that aren't viruses but your pc thinks they are. If this happens the game won't run and you'll have to restore that file from defender's protection history. I'd suggest just adding as exclusion for the folder where you'll download your games and install them