I have run dnn for several years using old version (only up to 3.2.2 ) but thought I'd try to update my sites and all the modules to run 4.X.  After many hours a day for many days I am completely at a lost - I can't believe it's this difficult.  I've tried so many things - installed/uninstalled then updated/added users / access/securitties, different versions 4.6, 4.7, and now 4.8, Visual Express 2005, Visual express 2008, XP, Vista, blah blah blah - I don't even know anymore what is installed on my machine.   And after every combination and reboot I get the same problem over and over.  Searched DNN and the entire web and tried every solution.  I have never been this frustrated with a simple installation of anything. 

I don't even know how I can reset my machine to start over.  All I want at this point is to install a recent version (4.8?) and get it to run in the base mode w/o any mods. 

EVERY INSTALL ends up with "The website is unavailable"  and from what I can tell (which isn't always obvious) its because the SQL connection string is not working probably due to User Instances not working.    SQL Server express has been reinstalled many times and I think at this point everyone has complete access to every folder on my computer.

Back to 3.2.2 wtih VS 2003 - simple and working!

You should be able to update your site easily. Please list out the steps you are taking to upgrade and we can let you know if there are any missing steps. Also, don't forget to back it all up before you try to upgrade.