Colleen,
I tested on a v10 SP5 machine and successfully Calculated Geometry for the X and Y coordinates of the centroid with a polygon feature class. There seems to be something going on with either a) your data or b) your current mapping system. You may try to:
1) Use the Feature Class to Feature Class tool to make a copy from your polygon layer (to relieve potential corruption within the file)
2) Upgrade to SP5 (you can check which SP you have in ArcMap by going to Help > About ArcMap)
Other things to consider are:
1) What field type are your x and y fields?
2) Where is your data located? (move it local if it is on a network or shared drive and then test it)
3) Are you working in a specific saved MXD file? (open a blank, fresh instance of ArcMap, drag in your one polygon layer, and test the performance, to relieve MXD corruption)
Hope this helps!
Matt Payne