check your Documents/EA Games/The Sims 2/Config folder for a file named userStartup.cheat. If you have one, open it with notepad, and look to see if "boolprop snapobjectstogrid false" is listed. If it is, turn it to true. If it's not listed, write it in it's true state anyway.
If you do not have a userStartup.cheat file in that folder, you will have to create one using notepad, just make sure notepad doesn't automatically add a .txt extention after the .cheat extention.