Store items and cc not installing is a problem that I have had since way back in patch 1.8, but I have finally found a way to completely fix this issue, thanks to a nifty little program I found. So I'm sharing the solution.
Please note that this method changes the content in such a way that it is classed as a completely different item. Because of this any saves using the store items/cc (that were previously installed but no longer work) may not work properly after doing this. To get around this run the game, load the save and change all the items for default ones. This lessens your chances of it breaking your saves. Also back-up the save folder, just in case.
1. Download the TG Unblocker:
Mirror 1: http://www.megaupload.com/?d=6QFXQGP5
Mirror 2: http://www.4shared.com/file/AzqSiPKn/TG_Unblock.html
2. In the zip is a folder called TG_Unblock, put this on the desktop.
3. Copy all of your downloaded .Sims3Pack files into that folder. If the items are from the store you will find them in the Sims 3 save folder in a folder called Downloads.
4. Run the Unblock.exe. Wait until it is finished and press any key to exit the program.
5. Copy all the .Sims3Pack files into the Downloads folder in the Sims 3 saves folder (Usually My Documents\Electronic Arts\The Sims 3\Downloads).
They should all now install successfully via the game launcher.