You should be able to use windows add remove programs as normal to uninstall.
It may be best to do a clean reinstall, ensure that you have your Pro license that was emailed to you available as you will need that once installed.
Download the latest Pro version of avast 4.8.1296 (if you didn't previously save it) for the language you use and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility, find it here
and save it to your HDD.
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, 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.