This is normal behaviour for Avast to run the scan after installation. It's called a boot-time scan.
The idea behind this is that the scan is run before the operating system fully loads. 99+ % of malware need the operating system loaded to wreak havoc. If the operating system isn't fully functional the chances of malware detection and removal is far greater. So the boot time-scan is to give the AV program (Avast) the best possible start to protecting your system.
You don't have to worry though, Avast will only run the scan once after install. Every time after it will just load normally with the OS. So let it do it's thing just this once then you won't have to worry about it again.
If you ever feel the need to make Avast run the boot-time scan in the future you can do it from the menu in the GUI.