If the volume that the temp directory is on is not the same as the install path, then it won't let it install. See the lines in the source for it below:
https://www.google.com/search?q=set+temp+path+batch
To fix it you have to set the %TMP% in a command prompt and launch the installer from that command prompt.
set TMP=C:\Temp
set TEMP=C:\Temp
cd %userprofile%\Downloads
qt-unified-windows-x86-3.0.2-online.exe
Hope that helps.
For those who wants scripts only Setting the TMPDIR environment variable (Linux, UNIX) :
mkdir ~/tmp; export TMPDIR=/home/username/tmp;