What usually breaks my OS is my love for trying out new software. Deinstalls never get rid of everything, so eventually there are just too many remnants lying around. I don't think I've lost a single OS install to a virus or a real security issue. It's just a matter of too many programs bugging things down.

Using something like Delta might help with that, but the effort... might as well calculate an hour every few months to get a clean and pristine system. :-)