From Birnam Designs Wiki

Manually

You can refresh Windows' icon cache (e.g. desktop link icons) by deleting IconCache.db from your profile directory. (e.g. C:\Documents and Settings\Username\Local Settings\Application Data)

The cache file will be automatically recreated.

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Programmatically

You can refresh the icons programmatically by passing the message SPI_SETICONS. Set uiParam to zero, and pvParam to NULL.