I am new to qt. I am not sure what installation I should download and install.
I would like to develop desktop applications that run on both windows and Linux x86.
Would I have to download and install on windows to run my apps on windows. And also download another installation and install on my Ubuntu to run my apps on Linux?
Or could I just install on Ubuntu and compile my applications to run on both windows and Linux?
The source code will remain the same for the most part. You will have to compile the sources separately for Windows and Linux on the appropriate platform since the two operating systems use different binary formats. Also, note that you will need appropriate Qt dll/shared libraries to run on a given OS.
Theoretically it's possible to cross-compile the code, but life is much easier if you just compile it on the target platform. (That means once for windows and once for linux.)