A good solution is to separate your internal HDD in two partitions. You can install the system in one "OS" partition, and save all your documents on the other "Data" partition. When it is time for a reinstall you can just wipe your "OS" without need of copying back and forth your documents. (Keep a backup for them anyways.)
Also, you can save as many portable apps as you want in a "Programs" folder in your "Data" partition, and they will even save their settings in case of a reinstall. Most programs (but not Office for instance) will work OK also when installed to another partition, even after a system reinstall.
For instance, with a 500 GB disk, I've dedicated 80 GB to the system (and i'm using about 30 GB), the rest is my "Data" partition, where I've stored many programs. Also, if you make a backup of your "OS" partition once it is set as you like it, then reinstalls would be even faster.
Ask if you have any other doubts.