does it crash whilst closing down? This means that some of the part.met files get corrupted and when you next load up, the program can't interpret the data. There's a tool called the donkey doctor, this may be able to recover corupted files. It's in the download section on the main page.
BTW, surely using edonkey hybrid is a better choice than just overnet? Overnet maybe faster on occasions, but the network doesn't have nearly as many files as edonkey, so a client with both might be a better solution
edit: oh i forgot, has overnet screwed with the part / met formats like edonkey did? That might mean donkey doctor won't work.