Microsoft have announced that there are problems with the password protected file shares in these products that allow access via the internet that makes it easy for unauthorised visitors to open or delete files.
This does not affect Windows NT as a different password scheme is used and with Windows 2000 a password protected screen saver is not protected if launched manually.
The problem in Windows 9x and Me is only at 'share-level security' and not at 'user-level security' if the access controls of a Windows 9x machine is part of a Windows NT or Windows 2000 domain.
To find out further information, visit Microsoft's security webpages that can be found at Windows Security Pages.