Once 30 days activation period is over windows xp won`t start,halting at logon screen.This can be miserable,but donot worry.It can be activated after 30 days and also can be used without activating.Wanna know how
Before booting press F8,which takes you to the boot menu.Select Safe mode with command prompt option from there.Once this is done it will boot windows.Select adminastrator account and you will see that you no longer get activation dialog box.
You will see that windows boots without any taskbar.But donot worry just type this line in comand prompt on the screen
C:\windows\explorer.exe
It will load explorer and hence your can browse you files and can activate windows using whatever method you like.
This trick works for me with windows xp service pack 2
This works. I was surprised it did, because I couldn't boot into any other safe mode except for command prompt, and I was able to activate windows from there.