I have downloaded the game several times and each time get the message "Installation failed" I have disabled my antivirus. Whats wrong? Help! please anyone.

Four things to note:

1. You *must* place all downloaded installation files into the same folder.
2. The install directory must have full access privileges.
3. Your %TEMP% folder must be on a drive with plenty (Gigabytes) of free space.
4. You must not install to 'Program Files' or 'Program Files (x86)