Well after a period of time avast would stop giving update notifications for avast5 (assuming you missed the initial ones) as avast5 hasn't had a program update in some time, e.g. since avast 6 was released. We also don't know what version of avast 5.x.xxxx you were using, if that was a beta build, that to could be a factor.

I don’t remember the exact version, but I’m sure, that it wasn’t a beta version. Avast 6 has also did this.

I also don't know if you have set the Program update to Manual, in which case you wouldn't get any program update is available pop-up.

No, “Offer me when a program update is available” is selected. I told this in my subject above.