New pc builder help over 20gb used on new ssd?

Hi, I recently just built my first computer. I decided to use Win8

Specs:
cpu: i5 3570k
MB: AsRock z77 extreme4
memory: g.skill 8gb ram
videocard: gtx670 ftw
storage: corsair force 120gb ssd
WD black 500gb
PSU: corsair hx750w
Case: CM storm scout

TLDR: 20GB used on new SSD? Everything works, do I still DL drivers or would the drivers cause conflict?

I had some troubles while putting together this computer. It would not power on. After trying many thing taking it apart and testing each part individually. I spent probably 20+ hrs figuring out what the problems was. I noticed that the AsRock MB is curved. When I screwed in the MB onto the mount it straightened it, so it could not power on. So I left the screws on the mount very lose, so it can keep it's natural curve. Would that cause a problem?

Once I finally got the system to power on, I install win8. I thought it was strange that my newly installed win8 already had sound and was able to use my wireless internet from the wireless card that I installed on my pc. I did not install any drivers. Does win8 already have all the drivers? Do I still need to DL all the drivers? or would it conflict with WIN8? Also I noticed that my SSd already used up 20+ gb already? What is going on? It's all so confusing to me.

Sorry for asking so many (wierd/stupid) questions, I am very new to building computers.