Greetings to all at the forum.
I hope that I can adequately describe my problem - here goes!!
My PC has Ubuntu 14.04 lts as its only operating system after getting rid of windows xp.
The system boots perfectly, giving me a screen with appropriate wallpaper and vertical directory bar on the left hand side. Most of the time I can open firefox, surf the net, consult emails, exit & close down the system without any problem.
However, If I click on the "Ubuntu" button on the top left hand side (is this the dash?) havoc breaks loose on the screen. Horizontal lines of different colours appear & the vertical directory bar dissappears! A diagonal line from bottom left to top right breaks screen into two parts. Sometimes an upsidedown image of my wallpaper appears in one of the sections. The screen flickers with coloured rectangles as well. A couple of times there has been totally black screen with the cursor (which moves but does nothing)
I hope that I have been able to give a general idea and that some kind person can advise!
I'm looking forward to getting this cleared up once and for all!!
Thanks very much.

This type of behavior is often displayed with some video cards due to the drivers. In most cases, it happens on boot not after clicking the Dash so I don't know if this would help. Go to System Settings, Software and Upgrades and you should have an Additional Drivers tab. Open that and make a note of the current settings and see if you have an option for proprietary drivers and install that. It takes some time to do it so you may have to wait a minute or two. Not sure that will work in your circumstance.

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Yeah, I have seen this myself. I ended up fixing it using Yanceks post advise.
Just a guess. You have a nividia chip (probably legacy) and you are using the nouveau open source driver.
I ended up making me a vesa driver /etc/X11/xorg.conf file to tide me over till I got the Nvidia
proprietary driver installed.

I keep it here.

I ran into that testing AntiX. So hence the name of the file.

I had a similar problem when I installed Ubuntu 14.04, using Nvidia Graphics Card. The usual method to get on is add 'nomodeset' to the linux line while booting, then doing the Additional Drivers thing. I went through this process and rebooted and it booted to the login screen. All OK, I though. I don't use Ubuntu often so the next time I booted, same thing with the dashed lines. Checked that I had the Nvidia drivers and I did. It was curious because every time I did a cold boot with 14.04 this happened. I could not do anything but power off manually and if I rebooted then it took me right to the login screen and all was OK. Tested this a number of times. It works now, maybe it just needed a rest as I don't boot it very often.

Thank you very much for your advice Yancek & Rokytnji.
I checked out the additional drivers in the system settings and I was using X.OrgXserver - Nouveau display driver.
I chose the NVIDIA legacy binary driver - version 304.117
After rebooting, all seems to be functioning perfectly.
So, I think that I can say problem solved.
Thanks again for your help!
Best regards.
David

ps do I have to indicate anywhere that a solution has been found?

Glad it worked. I wasn't sure based on your initial post if that would do it.