Haven't been able to play The Sims 3 since i built my new pc in december

neoromanempire​  I'm not sure why the registry didn't update, but there's no reason this shouldn't work as a location for Documents.  It may not be the best idea if other people use your computer, since this folder would be accessible to them, but if you're the only user, that's not a concern.

If you still can't play, change the registry value for Personal to C:\Documents, restart again, and test the game again.  And make sure the registry value didn't revert somehow after you restart.