why does alias in apache not work?

antelope

I've installed apache, and nearly changed nothing on the default conf file of apache,
I can brows the page in "docomentroot", but when I try to brows "http://myip/icons", it cannot be reach, the error is
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Object not found!

The requested URL was not found on this server. If you entered the URL manually please check your spelling and try again.

If you think this is a server error, please contact the webmaster.
Error 404
192.168.1.64
Apache/2.0.47 (Red Hat Linux)
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I never change the config concerning "/icons/" in the deafault file, why It dosent work?