When you say "Register" are you talking about Product Activation?
Registration can easily be disabled by including the [StartupOptions] in the Sysprep.inf.
You can get more information from deploy.chm in the Deploy.cab that comes with Sysprep.
[StartupOptions]
Answer File = Oobeinfo.ini
The [StartupOptions] section contains entries for specifying startup options.
Sample
This sample demonstrates one use of the [StartupOptions] section of Oobeinfo.ini.
Entry Description
DesktopStartURL Specifies the universal resource locator (URL) of the initial page displayed for Internet service provider (ISP) sign-up that occurs after the user has completed Windows Welcome.
DesktopWindowHeight Specifies in pixels the height of the Windows Welcome window in desktop mode.
DesktopWindowWidth Specifies in pixels the width of the Windows Welcome window in desktop mode.
OEMAuditBoot Specifies whether to restart the computer in Windows Welcome or in Factory mode.
Offline Specifies whether to run Windows Welcome in offline mode.
Registration Specifies whether to display the registration pages.
ScreenResolutionCheck Specifies whether Windows Welcome detects screen resolution.
[StartupOptions]
DesktopStartURL = "
http://www.fabrikam.com/signup"
DesktopWindowHeight = 530
DesktopWindowWidth = 640
OEMAuditBoot = 1
Offline = 1
Registration = 1
ScreenResolutionCheck = 0