If the install is failing during install, it could be timeout setting or IP address setting in IIS.
Avoid using "All Unassigned" for the IP address.
Check app-pool resource settings (number of transactions, kernal request, etc..) these may need to be unlimited for DNN installs, particulary 3.3 and above.

problem; no matter what i do, changes to the web.config do not help!
Well, yet another saga in dnn troubleshooting.
The casit website went down, and multiple people were trying to fix it. (problem 1)
root cause, the sql config was wrong, but someone tried to enable the errors mode by pasting customErrors mode="Off" into the top of the webconfig.
This is a major no no because that item is already in the web config. to make maters worse, they pasted at the top of the web.config ;




this effectively disables every entry below the
tag!
Fix:
If this happens, restore a copy of the web.config from previous versions and start over!!!