I've been experiencing many problem lately since I fresh installed Ubuntu 12.04 LTS
on my SONY VAIO VPCEH with Windows 7 previously installed.

Sometimes when I try to boot Ubuntu, it gets stuck at loading screen.
There seems to be some problem with driver (as far I can judge).

Following is the log generated, when I press Esc during the boot (before the
problem occurs):

...
* Stopping System V initialization compatibility        [ok]
* Starting System V runlevel compatibility              [ok]
* Starting crash report submission daemon               [ok]
* Starting automatic crash report generation            [ok]
...
...
* Starting LightDM Display Manager                      [ok]

Nothing works after that, no Esc, etc, except restart.

Also I've observed the following:

• Inactivity of Hard-drive (Led doesn't glows).
• Flashing, or blinking, of Caps-lock and Scroll-lock

On restart, Ubuntu seem to load successfully. However, the loading
screen has somewhat basic graphics.

This problem started after I installed Additional drivers:

NVIDIA accelerated graphics driver  

Also, most of the times Ubuntu loads without any problem. However, it
is annoying to restart everytime it fails.

So my question is:

Why this happens and what is the solution?