Have you rebooted since this stalled update happened ?
If so what version of avast is it saying you have (?), that would give a clue if it completed.
If an installation was corrupt it is likely to show itself in the form of errors displayed, interface not working as previously, etc.
You may be best to do a clean install (this is for the avast free version):
- Download the latest version of avast, 9.0.2013
http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, avastclear.exe
find it here and save it to your HDD.
- 1. Now uninstall avast! - using add remove programs, if you can't do that (as appears to be the case) start from the next step, reboot.
- 2. run the avast! Uninstall Utility, it will request it be run from safe mode (and will handle the boot into safe mode), once complete reboot into normal mode.
- 3. install the latest version, reboot.
Note: save the copy of avastclear.exe so it can be reused as some users are reporting that after running it, it is self-deleting.