Either the throttle is sticking open (over tight throttle cable, sticking cable, sticking butterfly) or you have a monumental vacuum air leak. On the SPI cars this is often one of the small pipes to the throttle body coming adrift.
The idle on modern cars is principally a function of the ECU, so it should never be necessary to play about with any screws on the throttle body. In fact, 99% of the time, it's a truly rotten idea.
This post should go in the Cinquecento Seicento section. This isn't a matter of administrative convenience, rather, this section is for general FIAT chit chat and you'll get a better, faster, response by posting in the right place.