My safe mode isnt working?

Ok I have a mini dell(really old) that has no cd/dvd drive. Recently a fake antivirus virus has gotten on it and hasn't been letting me do anythng. I tried goin to safe mode to download somethn to scan it so I can remove this thing but when it starts loading the sys files it stops at a certain one then freezes. I wanted to put a new operating sys on there but the only way I know how to do that is by putting one on a flash drive which I don't wanna do. In general I just need to know if I can just put a new os on there and do a clean install to pretty much erase everythng. Or how to fix safe mode to erase that annoying antivirus virus. Id rather know how to put a new OS than fix safe mode but either one will help