Uninstalll the old version before McAfee. If you want to be safe, just disconnect from net. A lot of AV's can get picky if they detect another AV already installed and cause further issues down the line. If your behind a router, its basic security features will protect while you install from the outside world, if your PC is already infected, it doesn't matter too much.
Was once caught out being on line without AV in between installs, but it was a long time ago so Routers probably better protected now. There again, if it's not connected, no problem!