Mail 'Entire Message' disabled

[Derek Chesterfield]

When I enter a search term in Mail [Leopard] the scope options appear.  
But the 'Entire Message' option is greyed out !!
Using the Spotlight menu, I can successfully search for emails by  
content, but not from within Mail. I also did an mdimport of my Mail  
folder, but that made no difference. Restarting the Mac also makes no  
difference.

I don't have this problem on another Mac.

Any ideas how to fix this?