If you encounter the following error using PHP under IIS: "Warning: session_start(): open .... failed", you need to grant "read/write" permissions to the "IUSR" on the directory listed in "session.save_path" (in "php.ini"). 

In Vista, do the following:
1. Right-click on folder defined in "session.save_path" in "php.ini" file. (ex: "C:\Windows\Temp")
2. Select "Properties"
3. Click "Security" tab
4. Click "Edit"
5. Click "Add"
6. Click "Advanced"
7. Click "Find Now"
8. Select "IUSR"
9. Click "OK"
10. Click "OK".
11. Under "Permissions for IUSR", ensure the following are checked:
- Read & execute
- List folder contents
- Read
- Write
12. Click "OK"
13. Click "Yes" to change the permissions
14. Click "OK"