End Background Tasks
1. Hold down the Ctrl, Alt, and Delete keys at the same time
2. Click the "Processes" tab.
3. Click on the gray bar in the middle column that says User Name to sort by user name.
4. Do NOT select anything for SYSTEM, LOCAL SERVICE, or NETWORK SERVICE.
5. Click on anything besides EXPLORER and TASKMGR for this User Name or other user names.
6. Click on END PROCESS
7. Repeat these steps until only explorer.exe and taskmgr.exe are open for this user profile
Delete Files in Temp Folder
1. Double click on "My Computer"
2. Double click on the C: drive
3. Double click on the "Windows" folder.
4. Double click on the "Temp" folder.
5. Hold the "CTRL" key and tap the "A" key to select all.
6. Tap the "Delete" key on your keyboard and accept the "Are you sure you want to delete these files" prompt.
Clear Game Cache

To clear your game cache:
Go to The Sims 3 in your Documents folder:

XP: C:\Documents and Settings\"username"\My Documents\Electronic Arts\The Sims 3
Vista or 7: C:\Users\"username"\Documents\Electronic Arts\The Sims 3
Mac: \Users\"username"\Documents\Electronic Arts\The Sims 3
Look for any of the following files ending in .package and delete it:

Create-a-Sim Cache: CasPartCache.package
Build/Buy mode cache: compositorCache.package
Mod or custom content cache: scriptCache.package
New Sims and skin cache: simCompositorCache.package
Look for a folder that says Thumbnails. Delete anything inside of it.

Check the game at this point. If it still gives the same error you can clear your Worlds Cache:
NOTICE: Mac users will skip this steps.
Go back to your The Sims 3 folder and look for WorldCaches and delete anything inside of it.
To remove custom content or mods:
Go to The Sims 3 in your Documents folder:

XP: C:\Documents and Settings\"username"\My Documents\Electronic Arts\The Sims 3\Mods\Packages\
Vista or 7: C:\Users\"username"\Documents\Electronic Arts\The Sims 3\Mods\Packages\
Mac: \Users\"username"\Documents\Electronic Arts\The Sims 3\Mods\Packages\
If you wish to get rid of them completely, delete anything within the "Mods" or "Packages" folder.
-If you wish to save them, right-click on the "Mods" folder and select "Cut" and then right-click on your desktop and "Paste".



Above it says: "If it still gives the same error"
My error was that my game was crashing and lagging a lot.
Run in Admin Mode
I would suggest that you try running the game using admin access. For this right click on your game’s shortcut available on your desktop and chose the “Run as Administrator” option.
Patch Your Game
They sent me a link involving the Super Patcher linked here.