I guess my husband downloaded the wrong avast 
I just discovered I have avast pro and the trial version is up and so will not be getting any new definitions
can I just download the free version over it or do I have to delete it and then install free version
thanks
Download and save the free version http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe and the removal tool http://www.avast.com/uninstall-utility
Uninstall your current copy then reboot the system to safemode ( tap f8 while starting the system ) once in safemode run the removal tool for all versions of avast ever installed and once completed reboot back to normal mode and install the new free version.
If you need additional help, please look at the following video link:
http://forum.avast.com/index.php?topic=93544.msg915130#msg915130