While Windows XP detects drivers it gets to the point that it should detect the Plug'n'Play Monitor and then it asks the user if it should connect to Windows Update. If I just press no then it just continue and detects without any problems.

Do you guys know how to disable the question box?

Maybe this will help (don't know for sure, I never tryed it):

Source: ref.chm

DUDisable
Specifies whether to connect to the Windows Update site to download updates during Setup.

Syntax DUDisable = Yes | No 
Values Yes 
Instructs Setup not to connect to the Windows Update site.

No 
Instructs Setup to connect to the Windows Update site to download any available Windows XP Setup updates. Setup also downloads any necessary drivers that are not on the Windows XP CD-ROM.

Default Value Yes 
Example DUDisable = No

Comments Specifies whether Setup connects to the Windows Update site to download any available Windows Setup updates or necessary drivers that are not on the Windows CD-ROM. 
DUDisable is equivalent to the command:

winnt32 /unattend /dudisable

Setup disables dynamic updates by default so corporate administrators can more easily standardize on a known set of Windows system components.

EDIT: forgot to mention: put this under [unattended] in your winnt.sif