Hello yz747,
We have seen several posters report the same thing, that for some reason their long-time activated and validated installations of Office XP suddenly become Nongenuine.
Go to this MS KB article
http://support.microsoft.com/kb/903275 and scroll down to "Method 2". Run the FixIt in Method 2.
After the FixIt runs and you have checked by opening an Office program, restart and revisit
www.microsoft.com/genuine and click Validate Office once again. You should Validate normally and use of your Office programs should show no more nongenuine notices.
The above procedure almost always takes care of the problem.
Post back to tell us if it worked for you! :-)
For great advice on all topics XP, visit http://www.annoyances.org/exec/forum/winxp