Cookies On Localhost, Path & Expiry Issues

suppose i have a script which sets a cookie in this manner:

Response.Cookies("session_id") = '12345'
Response.Cookies("session_id").Expires = <future date here>
Response.Cookies("session_id").Path = "/"

and another script which clears it like this

Response.Cookies("session_id") = ""

The scripts are located in a directory "mysite" under iis root directory
and accessed through http://localhost/mysite/something.asp

When these are called repeatedly, everything works as expected
(script number 1 sets the cookie and number 2 clears it)

However, when I create a virtual directory called "mysite" and point it
to the same place, script number one is no longer able to set the cookie
after script number two has been called, even though they are still accessed
exactly the same way (http://localhost/mysite/something.asp)

But when I modify the code in script number one by removing the last two lines
("expires" and "path") it works as expected again.