Recently, an old machine running Windows XP SP3 had its hard disk replaced, since then the OS decided that the license was not valid and considered itself to be in a trial/evaluation 30 days period.
Since the activation/validation services of Windows XP got discontinued we could not reactivate the current setup using the internet.
We tried a few times to activate via phone but it failed as well..
Temporary Solution (mitigating the problem)
To give us some time to think about a solution, whenever Windows XP would say that the evaluation period was expired, we would boot into safe mode and then run this command rundll32.exe syssetup,SetupOobeBnk (in cmd.exe or in the Run option that is in the Start menu) that allows you to reset the evaluation period for up to 4 times (and if used properly will give you 150 days in total to find a solution).
Actual Solution
In the end, we used the installation CD to downgrade the installation using the repair option, then we tried the telephone activation again and it worked!
Although Windows XP was not complaining any more, that caused us some problems with the antivirus, as a shared system library (DLL) that was replaced was not compatible with the antivirus.
To resolve this issue, we booted the machine using a GNU/Linux live CD and replaced the library (DLL) from a backup that was in the old disk.
Warning: Downgrading the system using the installation CD most probably re-enabled old exploits and hacks making the system less secure.
Keep this information in mind as you cannot re-apply the updates to the system not even the security updates any more.