It's probably the map. There are currently a fair number of maps where something inherent to the map causes the demo to crash on playback.
If you want to post the map name (right at the start of the demo file), I can try it and tell you if it's the map or not.
CoH Codex : Demo Models/FX/MOVs : Demo Info
Arc 111022: "Doctor Geist and the Scientific Method"
Could be any number of things, but the most common one would be a bad map. Where was this demo recorded? Have you tried playing back a demo taken in another location, or just the one?
Anyway, there's really no good fix to the problem, but it's easy to diagnose. To see if this truly is your problem, all you need to do is remove the map from the demo. You'll be running around in a big empty sky, but the demo will run.
The simplest way to remove the map would be to use Zloth's demo launcher, just look under the "demo doctor" tab and it's there at the bottom.
You can also do it manually which is easy enough. Just open up the demo with a text editor (i.e. notepad) and within the first few lines you'll see the map info:
0 0 Map maps/City_Zones/City_01_01/City_01_01.txt
Then just take out the map info (make sure to leave the first bit):
0 0 Map
Then save the file and try running it again. This of course may or may not actually be the problem, but it's probably the most common. If it turns out that wasn't what it was, feel free to post back here.
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i used demo record when i was doing missions, one was to destroy some books:
maps/Missions/V_Warehouse/V_Warehouse_45/V_Warehouse_45_Layout_02.txt
another is an outdoor map to destroy the hive spires:
MAPS/MISSIONS/OUTDOOR_MISSIONS/OUTDOOR_EDEN/OUTDOOR_EDEN.TXT.
then there's this one:
maps/Missions/V_Warehouse/V_Warehouse_30/V_Warehouse_30_Layout_08.txt
i'll try out Relentless' suggestion now and post the results soon.
[added]
ok, that one worked, with the 0 0 map info. and just like you said, it showed me and the enemies running around in the air lol.
i'll try to avoid the "bad" maps next time then, thanks for replying and helping me out you two. 
Was there anything special about those warehouses that you remember? Usually it's just the more unique maps that have this problem (doc vahz's lair, the eden trial, etc.) but does happen on regular maps from time to time.
Also for the second map, you may be able to replace that one with eden (the zone, not the trial) and get it to work. In most cases, most everything translates over because the coordinates are exactly the same. Though I never tested myself to see if that map works anyway, so you might have the same result.
[ QUOTE ]
Was there anything special about those warehouses that you remember? Usually it's just the more unique maps that have this problem (doc vahz's lair, the eden trial, etc.) but does happen on regular maps from time to time.
Also for the second map, you may be able to replace that one with eden (the zone, not the trial) and get it to work. In most cases, most everything translates over because the coordinates are exactly the same. Though I never tested myself to see if that map works anyway, so you might have the same result.
[/ QUOTE ]
anything special? no, nothing really unique and it's just like any other warehouse map. i don't even think that there were EB/AV on those missions.
heeeelp! whenever i try to view my demo recorded files, the program would crash everytime forcing me to verify all the files when i re-start the updater up. ive tried using the demoplay tag, and even downloaded some of the demo viewing programs out there but it still crashes.
does anyone have any idea what i could be doing wrong that causes this?