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You can get by with creating only a large version of your icon. But you may not like the results when Windows shrinks it down. 
Icons large and small 
In 32-bit Windows, an icon really consists of two icons: one large 
and one small. Both icons reside in the same ICO file. The large 
icon measures 32 by 32 pixels; Windows uses it for shortcuts, in 
dialog boxes, and in Explorer when you choose to view files as 
large icons. The small icon measures 16 by 16 pixels; Windows uses 
it on the taskbar, on application title bars, in the File | 
Open... dialog box, and in Explorer when you choose to view files 
as small icons. 

When you're creating your own icon, Windows doesn't require you to 
provide the small version. Instead, you can create only a large 
icon, and Windows will shrink it in situations requiring a small 
icon. Be aware, however, that the shrunken icon's quality may not 
be high enough to meet your standards; it's often worth the extra 
effort to create both sizes. 
  

by Kent Reisdorph, TurboPower Software, KentR@turbopower.com 
Kent is the author of "Teach Yourself C++Builder in 21 Days" (SAMS 
Publishing).