It's 1999 and Inaho is the student president of a Catholic girls' school. When Mo-chan shows up and starts snacking on her classmates, Inaho stakes him and revives him, much to the dismay of Hono and Yuki. She then makes him teach History. ^_^; They have various wacky adventures; it's more episodic than the OVA and much more slapstick. Camille is now working for a less bug-eyed Earl, but the Rasputin/Hoshigami thing hasn't shown up (although I've only seen up to episode 8). There still are the weird monuments popping up, though. Another difference is that Inaho is after the O-parts for money, not immortality, and she's even MORE annoying. Personally, I prefer the darker, ecchier OVA. The TV series is rather silly, and there appears to be little to no development of the relationship between Mosquiton and Inaho - she's just completely oblivious to anything except money.
As for Mosquiton, he doesn't go berserk when he drinks blood anymore; he just fights well in his normal mode. Hono and Yuki are pretty much the same, but their little-kid personas are dressed in contemporary clothes instead of the same ones they wear as grownups. Basically, the TV series is like a somewhat diminished copy of the OVA, with the really interesting bits taken out. -.-;
The animation isn't as good (it's a TV show) and the music is the OVA music recycled. There are probably 26 episodes (the usual length for a Japanese TV show); I haven't seen them all yet. Grey Area, one group who subs Mosquiton '99, seem to be defunct or moved; I don't know where their page is now. There are other groups who sub the show, and you can get it from many distributors. Fansubs.net is your one-stop source for fansub information. ^_^
Episode summaries - be warned that these are pretty detailed, so if you don't want spoilers don't read them.