[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