a registry key could of been removed which could trigger this.
if your copy of XP is genuine this is not a problem.
click start, all programs and you should see activate windows at the top, or click on the pop up balloon which appears telling you to activate windows etc. it will attempt to reactivate your copy of XP, if it fails, you can click telephone where you will be given a freephone number to call microsoft on which will allow you to reactivate.
the majority of the time the call is automated, asking you three questions, after asking you to enter a very long number which is displayed on your screen.
1) did you buy your copy of xp, or was it preinstalled
answer: you bought it
2) how many computers is this copy of xp installed on
3) have you recently made significant hardware changes
answer: yes ( if you say no, it will not activate)
you will then be given another very long number in blocks of five, enter these in to the fields on the activation window.