This is something I have needed to do many times over the years, and it is quite easy when you find the tool that suits your purpose.
I suggest starting with this link: http://arcscripts.esri.com/scripts.asp?pg=1&sb=1&ob=asc&eDate=&n=&top=&eLang=&eProd=&perPage=10&eDesc=on&eQuery=Add+AND+XY&Submit2=search
These scripts will add 2 fields, and you will need to ensure your projection is set correctly, the accuracy of one of these that I have used is sub-millimetre so you may need to copy the XY into a new XY field will lower accuracy to avoid false detail.
The Arcscripts Website is a fantastic resource for these types of relatively simple queries that should really come as a standard tool.
There are options for adding an extension, or you will find at least one of them is an Avenue script. Forunately there are not many functions for A3.x that someone else hasnt already come across, I still use it occasionally when I havent got the time to sweet talk various files in ArcMap so they are "just so". But also many of these scripts will do the business for large gigabyte datasets.
Best of Luck,
ESRI & GIS Enthusiast.
Wellington New Zealand
Edited by Mapgyver on 11/15/06 02:02 AM.