I just upgraded from version 4.8 to version 5 and I get an error that ashbase.dll is missing and the program does not start
I tried repairing the installation, but I still get the same error. Now what?
Please try installing from scratch. Download you preferred avast! 5 edition here.
After the download, disconnect from the internet and uninstall your current antivirus. Reboot then install avast!.
now, when I try to install avast 5 it is asking for a license file
I do not have a license file, only a license key for avast 4.8
now what?
Are you using a Paid or Free version?
If you have avast 5 free, you dont need a license key when registering online
see page 8 here
http://www.avast.com/documentation/quick-start-guide-free-en-ww.pdf
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
there are problem yes http://forum.avast.com/index.php?topic=53948.0 
so this upgrade option (“install over”) is doesnt bring anything