Question: My Windows 7 setup uses around 16GB while Windows XP only needs around 4GB hard disk space. Seems weird. I use Windows only for gaming so I don't need a lot of stuff they have to offer.

What is the best way to reduce the size of Windows 7? What can I delete / uninstall and how? 

I'd also like to reduce the CPU and memory usage as much as possible (as long as it doesnt hurt game performance) - turning off all that fancy stuff and so on. What can I turn off and how?

Answer: 
You can [turn off Aero](http://www.howtogeek.com/howto/windows-vista/disable-aero-on-windows-vista/) by selecting one of the basic themes. This will reduce the load on your graphics card as Aero uses DirectX, which should improve performance while in Windows. It won't affect gaming performance as that uses full screen mode which bypasses Windows.

To reduce the disk space Windows takes up look at removing some components. On XP this was "Control Panel > Add/Remove Windows Components" - I don't have Windows 7 on this machine to double check if it's in the same place. As to what you could remove, I couldn't say. You'll know what you do and don't use.

Also, I've just checked in XP and there's not a lot you can remove and those items such as the games won't take up that much space.
Windows Vista/7 doesn't provide explicit way to disable hibernation. (I don't know why)
You can make this with following steps :

* Open command prompt with administrator privilege.
* Type this : powercfg -h off

If c:\hiberfil.sys file disappear, it has worked.