Thanks for your suggestions. I don't want the UAC alerts all the time. However I found some configuration settings for UAC, if we set it as "no prompt" by regedit, there will be no alert anymore for most applications that need approval. But the important changes for system area (for instance, changing HOSTS file) and some more actions (for instance, extracting an archive file to "Program Files", "WINDOWS", "Users", etc. folders directly) still aren't allowed. And these preventions occur automatically as well. So it is logical to use UAC in this mode after the necessary changes to the system by the user. Therefore I decided to use UAC with this way.
And I've found out the answers to my questions by myself. There is no splash window function of script blocker module of Avast on Vista. Hence, absence of the splash window doesn't mean that the module doesn't work.
I selected the option "Show detailed info on performed action" (in script blocker settings), and I got the information of script blocker actions at the bottom right corner when the protected mode is off (as I said before). But I didn't get the info of script blocker when the protected mode is on. So we can say that script blocker works when the protected mode is off, and it doesn't depend on the splash window.
We can turn off the protected mode by IE settings when UAC is on. People who don't want to turn off UAC but want to use the script blocker can turn off the protected mode by IE settings. Besides, I should remind that the protected mode can't be active when UAC is off (even the option is selected in IE settings).
Thanks for your interests.