You can use Software Restriction Policies or AppLocker so that they can only run the whitelisted programs and not random
AppLocker is easier to setup, but not available for all Windows versions.
It might seem like a drastic measure, but at the end of the day your parents probably don't need to install new software themselves. Automatic updates for programs can still work, if you set it right (for example with publisher rules).
Other than that: don't install software unless absolutely required (such as Java), use a PDF reader with JS disabled, disable macros in Office (if possible) and some other stuff...