My laptop fan is always active, even if CPU usage is really low (less than 5%). I have a Vaio laptop with Linux and Windows, but this problem only happens with Linux.
Sony Vaio Laptops
I have found an invaluable resource of information from this Google code page.
I suggest you start there and work your way around the internet using keywords such as ACPI. ACPI is what allows your OS to control things such as fans, power-state (hibernation and resume), screen brightness among other things.
Try installing lm-sensors via your package manager and run it via sensors -f. Your computer may simply not be aware of it's temperature, and is making sure it stays nice and cool.
Many Vaio models have somewhat recent BIOS updates what are supposed to fix such issues.
Words of Caution
You are in for a long read, as Vaio laptops have tons of problems with ACPI and similar functionality within Linux. Not saying it's impossible, but you will surely learn a lot by the time you find your solution.