Re: Please help. A serious error has occurred while loading Sunset Valley. New Game.
You could delete the Sims 3 folder in Documents by manually dragging it to the recycle bin; the game would be forced to spawn a new one the next time you tried to launch. There are plenty of causes of corruption in this folder, although it would be surprising if it was causing you problems before you ever had a chance to play. Still, one potential issue is that your currentgame folder could be corrupted. If you click on it and it won't open, that means the game can't open it either, which could cause the "serious error" message. (Side note: that folder should always be empty when you're not actually in-game, so if you ever do find files in there, you should delete them.)
Trashing your TS3 folder should get rid of a corrupt currentgame folder, but if that's not the problem, there's only one other potential solution I can find:
http://simswiki.info/wiki.php?title=Game_Help:A_Serious_Error_Has_Occurred
I think you tried unticking the "read only" box, but there's another fix to try at the bottom of the page, under "still not working?"