Can I delete the avast! startup files after installing?
Thanks…
Do you mean the setup (installer) .exe? If so, then yes. 
Best Regards…
If you mean the installation file I would keep it for a while I always keep copies of installation files of programs I download, obviously after a while it will become out of date.
But should you ever need to reinstall your system/start from scratch you can install it so you have at least some protection before going on-line to get the latest program version and VPS update, etc. So I keep a copy of my AV, Firewall installation files just in case.