I am using Windows 7 and my Dell laptop has stopped going into screen saver or sleep modes. I have tried modifying the startup, but don't feel confident enough to edit the registry as is suggested, but not clear why these would fail without any software being downloaded or other changes. How can I relive this or track down the source of the change

What happens when you click the windows symbol in the tray and on the right you see "shut down" ?

Next to it is an arrow. That will allow you to put the system into manual "sleep" mode.

Right click on a free area of your screen and a list should pop up. In it choose personalize. Then you can edit your screensaver.

I can enter sleep manually. Yes, I tried changing through "Personalize". It has no effect

Have you tried pressing the windows icon then choosing "help and suppot". Then type in the box "sleep mode".

Choose the option that asks about sleep and hybernation. There is a lot to look up on it and suggestions for fixing the problem. Too many to post here.

Yes. I ran through some of those suggestions in my initial question

Did you check your Bio's also ?