Sony vaio laptop win-7 64bit

The Fan is your Friend, it's telling you the display of webpages is overheating your cpu/gpu and the fan turns on to cool it down. I would be worried if it Stopped Working.

Have you cleaned out your cooling system lately?

yes

I'm going to take WR2's response a little further... Have you disassembled and cleaned it?
Depending on your laptop model, it may be very easy to access the HSF (about 18 screws removed from the bottom) - dust gets into the heatsink and becomes impossible to remove by compressed air alone which just blows the dust bunnies back to the fan, which then blows it back into the HS. By taking it apart you'll be able to find the dustbunnies and either directly blow them out or physically remove them.
There a a number of youtube videos that address disassembly of various laptops - find your model, see what is involved. If you feel comfortable attempting the cleaning yourself (some are easy, some very complicated), I would suggest that.

For the non savvy PC user, just take it to a tech. shop to clean it. Gentleness and anti-static play a great role for your laptop to work on re-assembly. Might be worth your money.

Also, don't place your laptop on a bed or anywhere it covers the vents. You can place it on a flat surface such as a desk or table.