I have installed Ubuntu 12.04 LTS using a pen drive on Alienware m17xr4. But the problem is it doesn't shows up at start up but when I tried to install it once again using the pen drive it revealed the previous previous installation.
Boot using a live usb/cd of ubuntu.
After booting with live usb/cd ,Run following command in terminal,
More information can be found here- > https://help.ubuntu.com/community/Boot-Repair
After running this program, it will show one url. Keep it with you. If the above method fails post that url here.