Do a phpinfo() and check the value of ext_dir. Then verify that php_mcrypt.dll exists in that directory.
Also verify that the main PHP folder (e.g. c:\php) has been added to your system's PATH environment variable.
If the error message is "access denied", try checking the file permissions of the .dll file - verify that the user that the webserver is running under (I'm not familiar with WIMP) has read/execution rights on that file (and probably the entire PHP directory while you're at it).