I had a prefectly functioning DNN but I needed a larger drive.  I backup up the DNN DB with SQL management Express and also backed up the whole DNN folder.

With the new drive I got a new install to work.  When I restored the DB nothing seems to have changed even though the management Express app confirms that my previously backed up DB is present now.  It still works up to that point although looks like a new install.  When restoring the DNN folder it refuses to work even though I took care of the machine keys in the web file.  The web file is the previously backed up one that worked with new machine keys.  It complains of a login string being incorrect but I checked and nothing out of the ordinary.

I have been looking for a backup / restore guide but could not find anything.

Please help

Just to expand on what Mitch said, you need to attach to the old db in the app data folder and then choose restore.  Point to the restore file and the db you want to restore then on the options tab, choose to overwrite existing database.

Paul