[PHP-WIN] why php_mcrypt.dll can not work in php 4.3 on Win2K+iis

[yuegong]

I download php 4.3.0 from php.net, zip file, more than 5M.
And my PHP dir is d:\php, the php.ini is also in this dir. I enabled gd2.dll and 
mcrypt.dll in php.ini , the browser display this message :
PHP Warning: Unknown(): Unable to load dynamic library 'd:\php\php_mcrypt.dll',
 
but the gd2.dll can work.

can anyone tell me why?

regards