and if using Avast pro?
then you need a key, that is what you have paid for, if you have a 4.8 pro key it will/should work on 5.0 pro
You can install the 5 on top of the 4.8 and it will the read the key from the 4.8 (this also will/should work on free)
But there seems to be some problems to the program after doing this, so i would do a clean install
remove the 4.8 with the uninstall tool before installing 5