package 'ubuntu-minimal' cannot be found

It seems like you're asking about two or three different issues.

The problem with your screen blanking out, but not when RhythmBox is run, could be a result of your system automatically turning off the screen, suspending, or hibernating. These actions can happen as a consequence of inactivity if configured to do so, which RhythmBox may prevent; furthermore, many multimedia applications deliberately prevent them from occurring because otherwise they would interfere with the application's usability. (For example, if you're listening to a song and the computer enters suspend mode, or you're watching a video and the screen turns off, that would be bad.)

You may want to check the configuration in System > Preferences > Power Management. Another possibility is that it is a blank screen saver automatically activating. To see if that is the case, you can check the configuration in System > Preferences > Screensaver. This is a separate issue from "package 'ubuntu-minimal' cannot be found", so if that doesn't solve the problem with your screen blanking out, please create a separate question for that, and include information about the relevant configuration in the Power Management and Screensaver settings. (If the problem is that when the screen blanks out, you cannot get it to display again, then please mention that as well!)

If the rippling on your screen started after the problem with the screen blanking, then it is almost certainly an unrelated issue, so you should start a separate question for that as well. Unless the cause is the screen getting old or being exposed to a magnetic field, this is most likely an issue with video drivers. If you post a question about it, people should be able to help you with that.

Let's use this question to deal with the problem with the Update Manager. I'll post again about that, shortly.

It would be a good idea for you to go ahead and report this as a bug, not just because doing so might improve Ubuntu and help other users, but also because doing so may reveal the source of the problem and thus a solution or workaround. For example, several days ago another user experienced a problem with the same symptoms as yours and reported it as bug 658172; her bug report was analyzed, the problem was found to be specific to her Ubuntu system and not to reflect an actual bug, and a solution, based specifically on the information she furnished, was provided.

To report this as a bug, first, if you still have the original Update Manager window (with that message) open, keep it open. That will help in reporting the bug. However, if you have closed it, that's no big deal.

Read https://help.ubuntu.com/community/ReportingBugs carefully. This explains how to report bugs. The additional information that I am about to provide does intersect a little bit with the information on that web page, but is *not* a substitute for carefully reading through that page. The information I am about to provide also may or may not make any sense if you have not read that page.

If you still have the Update Manager window open, run ubuntu-bug with the PID of the process update-manager. If you don't still have it open, just run "ubuntu-bug update-manager". In your bug report, make sure to attach the requested files. In this case, those files are the files inside the /var/log/dist-upgrade folder. You can find those files by clicking "File System" in the places list on the left (either in any Nautilus window, or in your web browser's dialog box when you go to attach the files to your bug report). You'll see a folder called var. Enter that folder, and you'll see a folder called log. Enter that folder, and you'll see a folder called dist-upgrade. Enter that folder, and you'll see the files you should attach to your bug report.

After you have reported the bug, please link the bug and this question to each other by using the green "Link existing bug" link at the top of this page. However, your bug report should be self-contained; it should not be necessary for Ubuntu developers to refer to this question, in order to understand what happened when you attempted to upgrade from Ubuntu 10.04 to Ubuntu 10.10.

If you have any problems reporting this as a bug, please feel free to post again.

Thanks Eliah, in response to clarify:
If I don't start music player soon after switching on pc the screen flickers and blacks out, then I have to manually switch off and switch on again. Have lived with this problem for awhile until recent ripples started, which seem to indicate that the problem has worsened. Nothing wrong with the monitor which is quite new.
Need reasonably simple instructions to fix as I am not advanced enough for complicated terminal instructions but can attempt basics.

As I said, your screen blanking problem (and the ripples) are probably totally separate from your problem updating from 10.04 to 10.10. Therefore, please start separate questions about those problems. I won't necessarily be the best person to help you with them, and others might get a chance to do so before I do, but if you post links to the new questions here, then I'll be sure to see them.

In this question, let's get back to the Update Manager (10.04 --> 10.10) issue. If you have trouble understanding the instructions in https://help.ubuntu.com/community/ReportingBugs, I'd be pleased to help, but you'll have to tell me what you're having trouble with. (If it's a lot of different stuff, that's OK...just try to be specific.)

On the other hand, if you're having trouble understanding my instructions after the https://help.ubuntu.com/community/ReportingBugs link, then that's probably because you haven't yet read https://help.ubuntu.com/community/ReportingBugs (or have, but haven't understood it thoroughly).