If the agent is running as a service or a daemon, its web interface should be accessible at http://hostname:62354.
If the machine connecting to this interface is trusted (see httpd-trust configuration directective in agent configuration), a link will be available to force immediate execution.
Agent can also be executed from command line, using either of its multiple executables (see agent man pages for details).
Open a command interpreter windows (cmd.exe), with administrator privileges (right clic → Run as Administrator on Vista and Seven Start menu).
Go in the c:\Program files\FusionInventory-Agent\perl\bin (adjust this path depending on your configuration) directory and run:
# perl fusioninventory-agent