is_dir: runs bad under IIS/Win2K

Hi all! I have a little (or not?) problem with PHP filesystem functions with a IIS/Win2000 installation! I'm trying to execute a simple << is_dir("c:/tmp") >> but the function return 0: in practice for PHP c:/tmp doesn't exist. The same, obviously, with "c:\temp", "/tmp", "\tmp", "c:\\temp" and so on... The real strange behaviour regards the "session_save_path" directory: in my PHP.INI I've always set this parameter to "/tmp" (under Linux and the same under PHP/Apache/Windows, PHP/IIS/WinXP) and ALL runs well (without specifiying the unit X:\). Under Win2K, PHP seems to "consider" this parameter, in fact all SESSION FILES are written under C:\TMP. But when I try to check if this path exists (under Win2000), as so: << is_dir(session_save_path()) >> ... uhmm... the function fails! Any idea? Can anyone help me? Many thanks. Francesco