Thank you very much to Humberto Perotto and Andrew D. Mellinger for their suggestions.
 
I did the following to solve the problem.
 
1. converted all the data sets to grid with same projection and cell size.
2. used "combine" in Patch Analyst to combine world boundary and slope datasets - saved attribute data as table1 .dbf
3. used "summarize" to summarize grid coverage generated in 2 and population data sets - saved the result as table2.dbf
4. joined table 1 and 2.
 
My original question was the following.
 
> Population of countries by slope
> 
> I have two grid layers (population and slope) and one vector layer
> (countries). I would like to prepare a table showing countries in the row
> and slope in the column and population data in the table. In other worlds,
> what I want is population of countries of the world by slope classes. I
> know, how to do it if all the coverages are in Vector and I don't want to
> covert the population data to vector ( it is too big!). Is there an easy
> way
> to do this. Will sum. Thanks.
> 
> I have access to both ArcView 3.2 and ArcGIS (latest version) and I have a
> question.

**************************************************************
 Dr. Chandra Giri             
 Staff Associate (Research)           
 Center for International Earth Science Information Network (CIESIN)  
 Columbia University               
 P.O. Box 1000                
 Palisades, NY 10964                     
 USA                       
 Tel: 1-845-3658963                      
 Fax: 1-845-3658922                      
 Email: cgiri@ciesin.columbia.edu                              
 Home-page:http://www.ciesin.org                                   
          http://sedac.ciesin.columbia.edu                   
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