[horde] Mailbox.php not opening??

[NightHawk / Sean C.]

Hi all, I'm having a strange problem that I'm not quite sure how to solve... I downloaded the latest releases of Horde and IMP today, installed, configured, and when I go to my URL and type in my username and password and log in, it appears to log me in fine but when it auto-redirects to mailbox.php IE gives me the "open/save/cancel" option as if I'm trying to download the PHP file.

Anyone had this problem? Anyone know what to do? I have PHP and Pear and everything running fine on my server, I run other PHP apps just great... So I'm really quite at a loss. 

Thanks in advance!

Sean