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: 
4GB XP? 16GB Win7? That seems an awful lot. My vanilla win7 install on VirtualBox was around 5GB, and you can certainly get XP down to 700MB or so.

Use a visualisation tool such as [WinDirStat](http://windirstat.info/) to find out what's taking up all that disc.

Stuff to target, if you don't need it and you really know what you're doing:

* unused Windows components (Control Panel -> Add/Remove -> Windows components, or in Vista+, All Control Panel -> Programs and Features -> Turn Windows features on or off)
* System Restore
* \Windows\System32\dllcache on XP
* \Windows\System32\DriverStore\FileRepository on Vista+, if you're sure you're not going to need any more drivers out of it
* \Windows\SoftwareDistribution\Download
* %TEMP%, the IE cache, the trash
* patch rollbacks (hidden $ folders in \Windows on XP)
* example shared media and \Windows\Web
* disable virtual memory to get rid of the pagefile (assuming you have enough memory to run swapless)
* disable hibernation to get read of hiberfil
* delete the large Chinese/Japanese/Korean fonts, if you don't use them
* all vendor crapware (if an OEM install) must be destroyed as a matter of course

For some of these on Win7 you have to take ownership of the files back from SYSTEM before you can delete them.
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.