how to uninstall server 2008 R2 features

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http://serverfault.com – my friend told me : {I cant uninstall any features that you install it to win server,For example he installed web server to win server but he could not remove it anymore}. is it possible to unable uninstall features after i install it? how can i do that? he installed the standard edition of windows server 2008 r2 (HowTos)