two options come to mind:
1) make such folder accessible to only certain users
right click on the folder, select 'properties', select 'security' tab, and edit permissions as you see fit. you won't need password for the folder, but instead for the user.
2) if that's not an option then try installing 'microsoft private folder' (or some other similar program)
it creates a password protected folder. haven't touched it in years so i can't say how flexible it is regarding securing existing folders or network sharing and such
EDIT: i see microsoft private folder is not windows vista/7 compatible, pity. give 'my lockbox' a shot.