According to the host, its an .exe(D3Demo.exe)
You need a pretty stout vid card for Doom 3, I had to do alot of tweaking to my 9700pro to get it working decent. The graphics are good, but the online play was disappointing, much like MOHAA. I played it the first 2 days i had it, now it sits and collects dust. I'll stick with BF1942 and Desert Combat.