Qt5 installation problem
I tried to install Qt5 via the download link and then tried to install Qt5 using the installation defaults where three Qt selections were highlighted (Qt 5.6, Qt 5.7 & Qt 5.8) and then started to install a very large download (9.97 Gib) into a C:Qt5 directory. All went well in the installation until a "qt.tools.win32_mingw530 execution failed" error showed up, and I had to cancel the install. I have 64 bit computer with Eclipse CDT plugin 64 bit installed and I believe there is a 64 bit minGw already installed to go with it. I would think there is a 64 bit Qt5 version available, but it seems that the Qt5 download is for a minGw 32 compiler. The installer doesn't seem to like the 64 bit minGw already installed. Any solutions are welcome.
There is no need to download and install several Qt versions in parallel. Also one Qt version may come precompiled with different compilers. Certainly you may install everything or whatever fits onto your harddisk, but the question is why to block so much of your harddisk especially when you are starting out with Qt.
In general the default settings are storing everything in a path c:\Qt as you metion. Also the MinGW compiler should be installed by default in a folder under c:\Qt\Tools. There would go also a Qt creator version, which would find all tools and compilers already in the right place.
Since you are reporting that the error came up during installation you must have installed Eclipse already prior to the installation process of Qt. I would assume then that the 64 compiler has not been installed in a folder used by Qt installer. There should be no reason for conflict between 32 bit and 64 bit compiler version of MinGW. The only thing I can imagine is that you have some traces, most in the PATH setting, of MinGW 64 bit in your environment. This might create some problems even though not very likely I think.
I would recommend installing only one Qt lib version for starting and ensure that there are no traces of MinGW 64 bit left in the environment.
Yes, the precompiled version Qt version for MInGW is targeting 32 bit. Certainly you can compile a 64 bit version of Qt by yourself. However, it is a question if you want to start that immediately. Personally, I would start out with the 32 bit version. In most cases this is sufficient anyway. With the 32 bit version you can generate a.pplication for both 32 and 64 bit windows.