Look for this line in the php.ini file
session.save_path = \tmp
Change "\tmp" to a writable directory on your server.
For example, if you're running Apache on C:\ drive, you will have a "htdocs" directory something like: C:\Apache\htdocs
Now create a folder under c:\apache\htdocs, e.g. tmp
So now you have c:\apache\htdocs\tmp
Finally change that line to
session.save_path = c:\apache\htdocs\tmp
(if that doesn't work try session.save_path = c:/apache/htdocs/tmp)
Hope it helps!