Just give the IIS_USRS group on your machine modify access rghts to the folder you want them to be able to write to. This is done by navigating to your folder in Windows Explorer, Right-Clicking on the folder and selecting the Security Tab.
Becareful with this setting because this does have security implications and opens your server to a larger group with write permissions. Make sure that only the folder that you want Anonymous users to write to gets the permissions setting.