Well I don't know if when you install 5.0 over 4.8 it carries forward any password if set (don't think so). But if it did carry it forward (with the registration information it retains) then you would need to enter that password to be able to change/reset it.
So if you now don't know that password or have forgotten it, the easiest way round this is to do a clean reinstall:
Download the latest version of avast, 5.0.462 and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
Download the avast! Uninstall Utility, aswClear5.exe
find it here and save it to your HDD (it has uninstall tools for both 4.8 and 5.0).
- 1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
- 2. run the avast! Uninstall Utility from safe mode, first for 4.8 if previously installed and then for 5.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.