It's easy-just rename your Documents/EA Games/The Sims 2 folder to 'The Sims 2backup' and run the game. The game will then generate a whole new folder containing all the original maxis neighbourhoods. You can then shut down the game and drag the neighbourhood folders you require* to your 'proper' The Sims 2 folder. Just be careful you don't overwrite your custom hoods. Delete the new Sims 2 folder your game generated and rename the original one back to 'The Sims 2'
*Riverblossom is G001, Pleasantview is N001, Strangetown is N002 and Veronaville is N003. Just in case you forgot.