The whole business of what to back up or what not to backup is very subjective. There are three main groups of hard drive contents which you may wish to back up:
1. the system (including Windows, installed apps, drivers user settings etc)
This is typically done by imaging the system drive to an external drive. It will save the effort involved in having to reinstall Windows, updates, applications etc but unless you have a large system involving many apps etc you may not consider this worth the time and effort involved. It is also difficult to achieve if you have a single drive system in which all user data is embedded in user folders deep in the bowels of Windows.
2. installable applications
Typically purchased on optical drives or downloaded, secure copies should be maintained in case the originals become corrupted.
3. personal data (docs, audio, video, images etc)
As mentioned under 1 this is often embedded in the user folders on the system drive - totally contrary to industry data processing practice. User data should always be stored on a separate drive (logical or physical) making it easy to back them up en mass on a regular basis and also keeps the system drive to a minimal size and less fragmented.
The time and effort dedicated to backing up is directly related to the degree of suffering which would be experienced if the data were lost. I have three pc's all of which have images of their respective system drives stored on the other two and all of which contain exact copies of personal data which is also replicated on two external drives - I have data and work going back more than 30 years stored on them so I can't afford to lose it!
I reckon the minimum you should back up is personal data which may be irreplaceable and it is much easier to do this if it is stored on its own drive.