when i tried to install ubuntu from bootable USB it couldn't recognize my installed windows 7 so it showed me like i have to erase my whole drive and then ubuntu can be installed. please help me with this ASAP.
Ok, This should not be a problem.
First try with
wubi which comes with Ubuntu 1404 LTS CD. Inside there
wubi icon can be found. Once you double click it, it will starts the installation as a normal windows program. But what this does is that it installs the Ubuntu on the system as a dual boot instead as a program inside the windows. Thats all..
When system rebooting it will give you list of OS to select.
In the Ubuntu terminal, execute the command line below:
Then reboot the system, then you could see the boot menu options including both windows7 and ubuntu.
Please refer to: