The Spreadcoin wallet is pretty unstable. That's why I'm not keeping my SPR on there until they say we need to. It is safer on an excahange. My wallet went on a fork the first time I DL it. The dev said delete every file and reinstall was the only option. Since I hadn't backed it up yet, I would have lost all my SPR if I had already sent it.
You should always backup wallet.dat (especially just after you add a passphrase), and regularly thereafter.
Usually you should not delete wallet.dat from the data directory, even if you have backed it up recently, similarly for the [coinname].conf file.