Check this option if you are running under Windows 64 bits. This option automatically updates paths accordingly.
This option shall not be confused with 64 compiler settings which will need 64 bits library and will generate a 64 bits simulation engine. Only the compiler version defines if the target simulation engine will be 32 bits (win32) or 64 bits (x64).
This option has been disabled in v7 and checked by default as Windows 32 bits is no more used. Besides, vsTASKER will become 64 bits native eventually.