Had a problem with Exchange recently which required me to fix the security settings for all the IIS virtual directories for Exchange 2007 web access.  To fix i had to remove and then re-install the Client Access Server role.  The following article outlines how to do this not just with 2007 but also with 2000 and 2003:

http://support.microsoft.com/kb/320202

To summarise, open command prompt and enter the following commands (One at a time):

• exsetup.exe /mode:uninstall /roles:ca
• exsetup.exe /mode:install /roles:ca

Cheers,
James