It is usually caused by some other software using one of the files that the installer is either installing or trying to modify/update. Most times it is the virus checker checking the files as they are being installed and clashing with the installer. But you said you disabled your anti-virus. If the error message occurs with any file that has "ft" in its name (like ftdibus.sys, or ftd2xx.lib, etc) then try unplugging any FlashScan/AutoCal devices (and any other USB devices that may use the FTDI USB driver set).
And/or try rebooting the PC and installing the software immediately after a reboot.
Regards
Paul