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?"
I honestly don't know how else you could try to fix this problem, other than editing the .sgr files as @roberta591 has mentioned. (I didn't have to do this, and I wouldn't know where to start, either. At this point, I myself would probably go through the whole process of uninstalling, cleaning the registry with a tool like Piriform's CCleaner (free), and reinstalling from scratch, but editing the .sgr files is certainly worth a shot. You can probably find them here: C:\Program Files (x86)\Origin Games\The Sims 3\Game\Bin but that will depend on where you told Origin to install TS3. Regardless, you're looking for a file within an application's folders, not anything in Documents.
