No Boot: - Could Not Read The Selected Boot Disk

because mine won't boot. I get the message: "Windows didn't start because of a computer disk hardware configuration problem. Could not read the selected boot disk. Check boot path and disk hardware. Please check the Windows documentation about hardware disk configuration and your reference manuals for additional information." Well, that sort of thing is beyond me. But first, my system and how I got into this mess:

Trying to get rid of the screen that prompts you on boot-up to select an OS and counts down the seconds, I went from My Computer>Properties>Advanced and unchecked the first box, the one that has a place to the right where you say how many seconds to display the OS choice. I had changed that choice to "0" many times, but eventually the choice screen always came back, so I got the idea of unchecking the whole box. Then--I forget exactly which part of Properties this was in--I found the two OSs listed. They were the same, except that the one that was selected had a few words after it that began with "No"--maybe "No selection"? (This problem of having to make a choice on boot-up started when the computer was new and something went wrong and a Dell technician had me reinstall the OS.) I thought maybe I could get rid of the annoyance by selecting the other OS, and did it.....