RE: Windsurfing spots near Rome, Italy
It depends on the wind.
Personally I know a bit about the spots North of the town. Fregene is closer, but the bottom is shifty sand so usually waves (if/when any) are not the nicest. In Campo di Mare you can get nice thermals during Spring and Summer. A few good wave spots for experts are between S. Severa and S. Marinella, ccomodating for different swell and wind directions. Just North of Civitavecchia, and about at the end of the 100 km. range you asked for, there is another good spot in S. Agostino, where Southerlies are stronger than in other places and nice waves may rise in the middle of the bay.
South of Rome I also used to sail in Torvajanica.
Rome is quite a big town and traffic is horrible; so choosing the spot is also a matter of which part of the town you live or work in, if you don't want to spend hours driving.
Unfortunately I do my sailing much farther North, in Maremma about 150 km. from Rome, so I can be of limited help. But I am sure you will be able to find better guidance by the buddies you will meet.