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: 
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.
1. Use [Disk Cleanup](http://en.wikipedia.org/wiki/Disk_Cleanup) frequently to clean out temporary things.
2. See where data is being spent with [WinDirStat](http://windirstat.info/) and look if you can clean something.
3. Remove programs and windows components you don't need from the Control Panel.
4. [Resource Monitor](http://edge.technet.com/Media/Windows-7-Screencast-Resource-Monitor-resmon/) is great for checking out what things are happening when your PC is idle, see it as an improved version of Task Manager that is meant for power users instead of system admins.
5. If you want to know more about a folder, you can Google for it, back-up if you don't know...