ci on the site root error,by on the Subdirectory OK.

[eluser]Unknown[/eluser]
Environment：windows server 2003 PHP Version 5.2.8
upload to the root directory of the CI,visit
http://www.lazysoft.cn/
show:
Your system folder path does not appear to be set correctly. Please open the following file and correct this: index.php

but when i upload cito it Subdirectory like "ci",visit
http://www.lazysoft.cn/ci/
show:
Welcome to CodeIgniter!

it's ok!
at this moment visit http://www.lazysoft.cn/
it's ok!

then I rename or delete the Subdirectory of "ci" visit http://www.lazysoft.cn/
it's show error:
Your system folder path does not appear to be set correctly. Please open the following file and correct this: index.php

============================================================================
who can help,and tell me how to do. thanks