You do not say how your PC is set up; but I do the following
OS & programs are on one partition; data is on a separate partition.
Similarly with my external HDD used for back up.
Data partition gets backed up daily(only the files that have changed.
OS & Programs partition gets re imaged on a monthly basis. A full backup on each occasion, so that in the event of a failure I just restore the latest stored image & then update any changes to say AV that have occurred in the intervening period.
I keep at least the last 6 months of OS/program images.