You are prompted to activate Windows XP or Windows Server 2003 every time that you start
the computer
http://support.microsoft.com/kb/312295

Be careful with your details - very specific details will provide those you
are asking for help from the right information necessary to properly provide
help to you.  Registration of Windows XP is (and always has been) completely
optional.  Activation of Windows XP (non-volume licensing) is (and always
has been) required.

Reading your post *as is* and making no assumptions - one could answer that
you do not need to register - so don't.

However - reading your posting and assuming that you have incorrectly
referenced the terminology being used and the actual requests have been to
'activate' your copy of Windows XP - and assuming your Dell came with
Windows XP and you utilized their method of 'restoring' the computer to 'as
shipped' -- you need to contact Dell for support, as their copy of Windows
XP in combination with the sticker they placed on the machine should have
allowed you to activate the system.

If Dell was of no help or any of my assumptions (such as it coming with
Windows XP at purchase time) were incorrect (possible - as you did not give
said information) - then you could 'activate by telephone' as opposed to
activating overr the internet (or could have during one of the many times it
obviously asked you to activate) and talk/talked to someone at Microsoft
*if* the activation did not take.

Last suggestion - beyond calling Dell (those who support the computer/OS if
it came with Windows XP at purchase time) or Microsoft for assistance with
this is to look at this article:

You are prompted to activate Windows XP or Windows Server 2003 every time
that you start the computer
http://support.microsoft.com/kb/312295

That may assist you.

The fine manual for my Dimension 4600 said to check the
answer "Not at this time".  I suspect that is what the fine manual for your
machine said.
Jim