I have the same issue sometimes, it is not my computer it is a little annoying glitch in the game and it dislikes installing patches, new content, anything really.
Are you sure that you are patched up to the latest patch? Once when I reinstalled for some reason it wouldn't install the latest patch and I had to patch up manually. If you are sure that you are patched up properly then I would suggest this
http://www.carls-sims-3-guide.com/forum/index.php/topic,7507.0.html then install only EA approved CC, by that I mean, Store items, patterns created using Create A Pattern and World created in Create A World before any 3rd party CC or MODs that you may use.
It may be that your game is having a tantrum like mine does or that a piece of bad CC is conflicting with downloads somewhere.
If you are not patched up, then you are able to manually patch your game and the best instructions for that is on the Sims3 official website.
Kindest Regards
KJ