This indicates that you don't have correct permissions set. When DNN install's it tries to write an installationdate to the web.config file so it can later tell if it was an install or an update. To do this it need's modify permissions on the web.config (the documentation states that you need read, write and modify for the root folder AFAIR - please ensure you have those permissions set otherwise you may get failures)

BTW, the other values we write to the web.config are security key's unique to your installation, so not having the correct access, also means that you are not utilising this either.

Cathal