If you have set up more than one user in OSX, there is a little icon in the upper right corner of the screen which is a black silhouette. Click it and you will see the user list. Drag to the one you want to use. If he has a password, you will have to enter it.
Remember, the first user is still logged in, so you have to log him out before shutting down.
Apple's OS X doesn't have a registry - the closest equivalent is the Library folder, which requires administrative privileges to make changes if it's the system's Library folder. Users have their own Libraries which they have rights to change.
You cannot run OS X on a windows machine.
According to an analyst survey in March 2007, the estimated installed user base of Mac OS X is approximately 22 million users (source linked below). That number presumably does not include pre-Mac OS X (Mac OS 9 or earlier) users.
This is the problem, no one really knows. Unlike Windows or OS X, Linux doesn't really have you "register" or phone home to anyone, so no statistic on how many Linux users there are is accurate. I personally estimate Linux users number at least as much as OS X users, though many will not agree with me.
The name of the computer can be changed in the Mac OS X System Preferences' Sharing section.
Signatures for the Mac OS X Mail application are changed from the Signatures section of Mail's Preferences.
Mac OS X is pronounced Mac OS Ten. It follows on from Mac OS 9 which was the last version of what is now known as the classic Mac OS and it saw the introduction of the new UNIX based OS so the Roman numeral X was adopted to mark the change.
The Apple computer actually runs the Unix operating system but this is hidden from the users by a very good graphical user interface. Apple call their operating system "Mac OS X".
Apple's Mac OS X. OS X Lion OS X Panther OS X Leopard etc.
The second name of Mac OS X v10.4.5 is Mac OS X v10.4 Intel.Mac OS X 10.4 was known as Mac OS X Tiger.
No you can not get xcode on any windows operating system i heard (even thought this may be in correct) that only MAC OS X users can get this.
The only customization you can do with the mouse cursor in OS X is to change its size. This is available in System Preferences, Universal Access.