The idea is to let windows compress files much like ZIP or RAR to give you more storage. While it may seem like a good idea, doing so on your main drive (Where Windows is installed) will render your computer unbootable (if done wrong).
For some reason Windows – which will not allow you to accidentally format your main drive – does allow you to compress your boot manager and system-files. Causing an unbootable computer. Luckily there’s an easy fix, well as long as you have created a system Repair disk, that is. But sometimes even that fail – then what?