I have to format the system HD, and I don't know how to keep all the presets in Cubase... Also, will all the plugins load normally if I just reinstall them?
It does. Every VST plugin and VSTi has it's own save feature. It's part of Cubase, so the presets of everything can be saved no matter what plugin/instrument it is. You can save them as single files for every plugin. You'll have to export that for every plugin seperately though.
If all the presets are in use in the projects, just save a copy of the project files somewhere else, like on the internet, a data stick, external HD or something.