Groups are equal in size
N = the number of cases in each sample group.
From the SPSS output for KW, identify the Mean Rank for each group.
Multiply each mean rank by the N for that group to get the Rank Total for each group.
Now, for any paired comparison of interest between two groups, calculate
d = rank total of one group – rank total of the other
Next calculate K = d
 0.8
N x √N
Now look up K on this table. If K is equal to or greater than the table value, the difference is significant
Total number of groups in the analysis 
When comparing any groups with each other in pairs 
When comparing several groups with a control group but not with each other 

If K is equal to or greater than the table value, then the difference is significant at the level indicated by p 

p = 0.05 
p = 0.01 
p = 0.05 
p = 0.01 

3 
2.89 
3.60 
2.72 
3.45 
4 
4.22 
5.12 
3.86 
4.80 
5 
5.60 
6.69 
5.00 
6.16 
6 
7.01 
8.30 
6.17 
7.53 
7 
8.46 
9.92 
7.37 
8.94 
8 
9.94 
11.58 
8.55 
10.33 
9 
11.43 
13.25 
9.77 
11.77 
10 
12.97 
14.95 
11.01 
13.19 
Table
adapted from R.Langley Practical Statistics Simply Explained Pan Books
1979 p220
Groups are not equal in size
One option is to run Oneway ANOVA in SPSS and use one of the Post Hoc options that caters for unequal variances, such as Tamhane.
Or one could use the MannWhitney test on each pair of groups and adjust the p value with the Bonferroni method, but that is rather overcautious and one might miss a sig result.
By the way, for k groups, the total number of possible paired comparisons = (k x (k1)) / 2.