Yes but it really depends where you download it.
Yes. Simply google "halo trial" and you will get several pages to download the demo game from. The demo lets you play on the campaign level Silent Cartographer, and the multiplayer level Blood Gulch.
As of yet, no.
Yes
i don't believe that there is a demo in PC only in xbox 360
Not yet but there most likely will be one for PC
I use cheat engine!
There is no demo on the PC
Yes, Download steam, make an account, search L4D2 and download the demo from there.
of the disk
Cnet.com
Yes, there was a Halo 2 demo shown at E3 2003. But after E3, Bungie scrapped the level and the demo because it could not be shipped on any gaming engine. After E3, nobody would ever be able to play this demo again. This has caused anger among many Halo fans at how bad the final version compared to the demo. Hopefully, it will someday be available on the Xbox 360 live marketplace, or at least with a special edition of Halo 2.
there is no way to get the whole HALO series for PC. although there is PC for call of duty