i just installed 12.04 on windows based Dell Inspirion 15 R (7520). The probelm is with the fan . They are running almost all the time . It is annoying and really bothering me. The system is not hot and the temperature for the 4 cores seems to be fine. What can be the problem and how to fix. I really don't want to go back to windows.
(Elaborating on my comment as an answer, to provide more info and in case it is the cause.)
Laptops with Nvidia cards and Intel processors come with a technology called "Optimus." What it does is dynamically switch between the onboard video (Intel graphics) and the discrete video (Nvidia card), depending on the graphics needs. This is designed to help save battery life, while still providing good graphics performance.
The only problem is that Optimus is not supported under Linux by default. As such, it can cause a number of different issues, usually obviously graphics related (screen freezes, generally). That's where the Bumblebee project comes in.
Bumblebee is designed to bring Optimus support to Linux. It's still a relatively recent software, so it's somewhat hit or miss as to whether it will work for you, but it's worth trying.
If Bumblebee doesn't work, there is also the option to turn off Optimus in your BIOS settings. Simply go into your BIOS setup/settings when you first boot your computer (before the operating system loads), find the setting for Optimus, and turn it off. That will then force the operating system to use only one of the graphics cards (usually the discrete one). Do note, however, that doing this will likely make your battery life shorter, due to the increased power requirements of running the discrete card all the time (how much shorter depends on what you're doing and how smart the discrete card is when it comes to power consumption).
You could configure your fans using i8kmon.
I posted a guide about it here on askubuntu: