Unable to install grub in /dev/sda

Exactly what command are you using to try to install grub, and are you running Ubuntu from an HDD or from a live-DVD? Please give information on your partitioning, and state exactly what you are doing.

I've downloaded Ubuntu of the website and burnt it to a disc, it goes through the whole installation perfectly until it reaches the end and gives the error message. I haven't change any of the partition and allowed it to erase all and use the standard installation. Thanks

Are you booting the disk or running it from Windows using the wubi installer? Do you have Windows on this computer and do you intend keeping it? If you are using the defaults and allowing it to use the entire drive, this might indicate a hardware fault.

Boot the DVD again and use the Disk Utility to look at /dev/sda Also use the SMART test feature in the Disk Utility to test your hard drive. Report back on what partitions you have on /dev/sda as you see them on the Disk Utility.

In a Live Session via the Disc, open the Disk Utility. Find what the drive is mounted as, after you mount it. You can mount a drive via the file manager.

Mine happens to be /dev/sda. When done, run
sudo grub-install [mount (ex. /dev/sda)]

Then,
sudo update-grub

You should be fine then. This is how the installer does it.