what version of avast do you have, 4.8.1201 is the latest version ?
Windows error 2 = The system cannot find the file specified, so there is certainly something wrong with your installation.
If you didn't first uninstall avast it may bump into the self-defence of the new 4.8 version of avast.
Try add remove programs to uninstall avast, this may well fail because of the setiface error. In which case you can use the avast uninstall utility.
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.
2. install the latest version, reboot.