I want to use Ubuntu 12.04 Long Term Support but during installation it showed me a message
Grub can't be installed.
how can I install manually.
Try with Boot Repair.
1 - Live CD
First, you'll need to boot from a Ubuntu Live CD.
2 - Repair the bootloader
To restore the Grub bootloader, follow these steps.
Open a terminal from the Live CD, and run the following commands:
After a few seconds, Boot Repair's main window will open.
Reboot to test everything out.
If everything went well, you should be up and running now.
Do you have the right version of ubuntu? A fellow of mine got the 64 bit version instead the 32 bit. We spent three days trying to install it ... on an 32 bit pc! Thinking we had the 32 bit version, we tried a lot of advises and recommendations until we look at sources.list file, in which is recorded the version we were using. I suggest you check this small issue before anything else. Regards.