Turning off your anti-virus software will probably not work, since both the installer and the client executable are considered to be trojans, so it'll be blocked as soon as you turn it on again (or even while it's off, because the scanner is off but the download protection system might be still on). The thing to do is to add the *\MTAInstall05.exe and *\MTAClient.exe files to the anti-virus' ignore list. It varies from software to software, but everyone has (or should have) the option to do so.