System Restore can have an adverse impact on avast, you need to disable the avast self-defence module before using system restore. It looks like you now have a corrupt installation.

Since you have done it the best option is to do clean reinstall of avast:
Download the latest version of avast, 8.0.1489 (see below as you don’t say which version you are using) 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, aswClear.exe find it here and save it to your HDD (it has uninstall tools for all versions).
    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), first for 7.x if previously installed and then for 8.0, once complete reboot into normal mode.- 3. install the latest version, reboot.

Direct download locations:
Free AV: - http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe
Pro AV: - http://files.avast.com/iavs5x/avast_pro_antivirus_setup.exe
AIS: - http://files.avast.com/iavs5x/avast_internet_security_setup.exe
Premier: - http://files.avast.com/iavs5x/avast_premier_antivirus_setup.exe