OK guys.....problem solved (I think).
I just got a response from the PMDG support team (Paul Gollnick, to be exact). He had mentioned several reports regarding the HIGHMEMFIX and issues with the SP2 (or, if you have it, the acceleration pack). The solution is to simply uninstall SP2 (or the accel pack), and then just reinstall it. Everything else can remain as is. I just started FSX and the 777 pax version appeared normally. Haven't done any flying yet, will first take my time adding liveries, add-on scenery, etc.
Thanks to everyone for their input. And a special thanks to Paul G.