I think you're misinterpreting something that is explained in the docs. The reference is to an optional argument that forces PHP to spread out the session files into multiple directories. There can be performance reasons for doing this, but it breaks the automatic garbage collection. Almost no one ever does it, and it should have nothing to do with pointing the session handler at one specific directory.
If your ini file says
session.save_path = c:\php_tmp
... you should be OK. If it says
session.save_path = '5;c:\php_tmp'
... you will not be OK.
See http://us3.php.net/manual/en/ref.session.php
The actual garbage collection process (which will not happen until you get a significant number of hits on your server, as the "firing button" is triggered randomly) should not really be your issue. It is more likely that the problem is with the session cookie, which is the key that lets the browser gain access to the old session data.
Where you do need to look closely is at the value of session.cookie_lifetime. It should be set to 0, which causes PHP to set a cookie that is valid only until the browser closes. Any other value will be the number of seconds for the cookie to remain active.
