That happens because you're using the free version of WinZIP. It's the same case encountered in WinRAR as well. They display an annoying ad before the program can be used. Unfortunately, it's impossible to remove that ad unless you're buying the application to unlock it or use an alternative.
Yes, WinZIP might look fancy, but there's a free alternative that works very well and it's called 7-Zip and it also doesn't interfere with the functionality of Avast Free Antivirus.