Besides the correct setting that DavidR mentioned, this has been already “reported”.

If I remember correctly, there was a comment from one of Avast developers about this. For Avast 6.0.1289, there was a change so “Avast Welcome” would be compatible with the future “Windows 8”, but then they discovered that it might “broke” the Welcome screen under Seven in certain situations (I think it was also a Windows Seven x64 edition).