By first downloading the new setup file before uninstalling.
Though there is no guarantee that this would resolve the problem as if the same condition exists it could happen in the new install.
Is the avast icon still rotating when the scan appears to stall ?
What does Task Manager show for processes using CPU % avast processes begin with ash or asw ?
#### Clean reinstall ####
First Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html 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.