I have been given an acer laptop with no os. It appears to have had a dual boot of windows 7 and ubuntu 14.04.However the boot system has been corrupted. Any thoughts.

Download an Ubuntu installation kit from http://www.ubuntu.com/download/desktop
Burn a DVD with it or create a bootable USB stick with it (instructions are on the lower part of the linked page)
and then boot that DVD or USB stick in the 'Try Ubuntu without installing' mode.
You should now be able to inspect the hard disk to verify the installed operating systems.

If you want to install Ubuntu, just reboot with the installer and select 'Install Ubuntu'.

Do you want the system to be a dual boot?
What is your goal here?
What happens when you try to boot the system and what do you want to happen instead?

I just want an ubuntu system. When I plug in the usb drive the drive is ignored,

How did you put the data on the drive?

Downloaded the distro and used start up disc creator.
is there no way to get it to recognise the usb drive. I have a nearly new laptop which looks like ending its days in a rubbish skip.. Thanks guys

You have to instruct the computer's BIOS to boot from the USB stick.
When you switch on the system, you should see on screen some message like "Press Fx to enter BIOS setup".
You have then to select that the USB drive has higher priority than the built-in hard disk.