I’ve accidently maximized Avast (latest version) on my XP Home SP3, but I want to have it back to ordinary window mode.
I cannot do it because there is no according button in the upper right corner.
I cannot drag the edges of that window with the mouse, either.
Now, how can I make this window smaler again, please?
Double click the outer edge of the UI.
How about just right clicking on the avast tab on the Task Bar and select Restore.
The outer egde didn’t work because it wasn’t “touchable”,
but the task bar option was ok… now.
It’s just that I don’t understand, why this option didn’t work before I posted my question.
I got absolutely no idea why it worked now.
Anyway, thank you folks. 
You’re welcome.
I am having the same problem, and there is no restore to press in my task bar, what can I do?
If you don’t have Restore, then it is in normal window mode whilst it could still take up the full screen, so you would need to try Resize. Even so you should still have the Restore, but it would be greyed out and Maximise would be available.
What is the screen resolution of your monitor ?
It could be in Full Screen view - Try using the F11 key as that toggles between full screen and normal view. Though having said that the task bar wouldn’t be present. Are you able to do a screenshot like those I have attached ?
Sorry about that, I hadn’t even retried that option since probably the 5 versions and didn’t realize it wasn’t there anymore :-[
At least the other option DavidR suggested worked 
Don’t have any option even in normal mode, the screen resolution is 1366 x 768.
I tried to take a screen shot but not possible, there are only 3 options for me to choose and no resize!
Unfortunately not being able to see what you are experiencing and you don’t say what the 3 options are, we are working in the dark. I can’t really suggest anything other than I have already.