there are plenty, gens, kega, ages, retrodrive, xega...
i recommend gens though, it's the fastest and the most compatible and has excellent sega cd support
both, Kega and Gens, work for me in xp. You can get Gens at http://gens.consolemul.com/ and Kega at http://www.eidolons-inn.de/Kega. I would recommend Gens too, but Kega is very good too, both have sega cd support and an very high compatibility, Gens is around 99 % for normal genesis games, I think.