I don't think that there is anything obvious that you have missed as that is where the scheduling is in the current avast version.
I'm just wondering if there is some data relating to a previous version of avast of this unknown scheduled scan, this could be in the avast5.ini file. Whilst this .ini file is a text file it isn't to easy to read and unless you know what your are looking at to be able to modify it safely isn't something I would recommend.
You could try an avast Repair, but I doubt this would do a clean of an old set of scan setting - so I would suggest a clean reinstall of avast. See below.
Before you do a clean reinstall ensure you have a copy of your avast pro license. If you haven't saved it you could use the - Resend avast License
http://www.avast.com/resend-license-paid.php, you will need to enter the email that you used when purchasing the license, e.g. the one that the original email with the link was sent.
Clean Reinstall.
- Download the latest version of avast, 10.0.2208
http://files.avast.com/iavs9x/avast_pro_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
http://files.avast.com/iavs9x/avastclear.exe and save it to your HDD.
- 1. Now uninstall avast! - using add remove programs/Programs & Features, if you can't do that 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.