Did you try uninstall Sitefinity and remove all files left after the uninstall process ends?
We suppose that the access rights for ASPNET were not set correctly during the installation, or there are old assemblies from a previous installation. To clear up the possible reason, please uninstall Sitefinity, delete all folders under the Sitefinity3.0 directory and reinstall it by leaving all settings unchanged. Please check the following after installation:
- You should have administrative privileges on your local machine
- Account <yourmachine>\ASPNET have full access rights for Program Files\Common Files\telerik\Sitefinity3.0
Also, is your virtual directory nested inside another? The problem might occur if there are web.configs which are messing up due to the virtual directory hierarchy.
Please, let us know how it goes.
All the best,
the Telerik team