I installed ubuntu 13.04 but system always starts windows 7 and no option for ubuntu.please help me
I'm assuming that you have installed Ubuntu by using a bootable USB. If you used a CD, just substitute every "USB" with "CD".
Okay, there are several ways to fix this. However, I like the following method because it gets you familiar with Ubuntu if you haven't used it before. This method uses a utility called Boot Repair, which tries to fix problems when booting Ubuntu and/or Windows.
Power off your computer. Plug in the USB you used to install Ubuntu. Power on your computer. Boot from the USB, as if you were going to install Ubuntu.
When it loads up, choose "Try Ubuntu". This will open up Ubuntu for you. However, this is not the Ubuntu that you installed, it's just Ubuntu LiveCD, which is running from your USB.
Connect to the internet.
Open up a terminal (Ctrl+Alt+T). Type in the following and hit enter:
When it's done, type the following and hit enter:
When it's done, a window should pop up, called the Boot Repair.
Hit "Recommended Repair" in the window that pops up. When it's done, note down the URL (paste.ubuntu.com/XXXXX). Then restart your computer (don't forget to remove the USB so that you don't boot from it again. I think at some point when restarting, it'll instruct you to remove it anyways).
If the above did not work, post that URL here so we can further assist you.
All the above information was taken from here.