First check to make sure that all the Eureka and webpublishing services are running.
Check the Virtual Directory set up in the web server, usually "myeureka", or "iqwp". Make sure that it is correctly pointing to the Report Server install directory, and that it has "execute" permissions, in IIS there is a bullet for "Execute(including scripts)"
There should be an IUSR_xxxxxxx (I.E.: IUSR_GATEWAY003 or IUSR_ATLLAND8) guest account set up as part of the Administrators Group and have FULL PERMISSIONS.
In regedt32: HKEY_LOCAL_MACHINE/software/MyEureka Report Server/CurrentVersion/MyEureka! Report Web Publishing -> with this folder highlighted, select Security/Permissions. The IUSR_xxxx guest account should be listed with full permissions.