Well, if the About box says it's 4.5, it probably is. If you want to do some additional checking, you can go to the program settings and on the page "Common", look for the checkbox "Enable Skins for Simple User Interface". If it's there, it's avast! 4.5 - avast! 4.1 didn't have this option.
Or, just rightclick on the avast! tray icon; is the 4th item "Program settings"? If yes, it's definitely the latest build of avast! 4.5.
So, now you can work and restart the computer normally unless you try to update the program... right? If you invoked the program update from the avast! menu... what happened? It said that the program has been updated, asked you for restart and when you restarted, the computer was frozen? Did you try to restart (manually) at that moment?
Also, please check avast! logs (Log Viewer). Are there any errors there? (e.g. in "Error", "Critical" or higher cathegories)?