I have purchased and downloaded Avast! 4 Professional edition at the end of my free trial period. When I try to install the process ends with a message “there was an error while changing the product configuration”. What to do? ???
All you should have needed to do was to enter the license key you were sent by email into the License Key area in about avast and it should have converted it to the Pro version.
Were you using the on-line setup and installation or did you download the full off-line setupengpro.exe file (about 38MB) ?
It may be easiest to do a clean reinstall:
Download the latest version of avast! Pro http://www.avast.com/eng/download-avast-professional.html and save it to your HDD (if you didn’t save it before), somewhere you can find it again. Use that when you reinstall. Ensure that you scroll down and select the avast direct download link for the English version and not Cnet as that is for an on-line installation (not what you want to do).
Download the avast! Uninstall Utility, find it here and save it to your HDD.
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- Now uninstall (using add remove programs, if you can’t do that start from the next step), reboot.- 2. run the avast! Uninstall Utility, reboot. If step 1 failed it may be necessary to run this from safe mode, once complete reboot into normal mode.- 3. install the latest version, reboot.
Sorted. Many thanks for the help. 
No problem, glad I could help.
Welcome to the forums.