HTC Desire :: Can't Play Video

I have tried to transfer an mp4 (Terminator salvation) from a laptop to my phone and when I play it, I get the 'cannot play video' message.I thought that I may have done something wrong when transferring so today I also tested it via Bluetooth with a friend who also has the desire but on o2 (I am with Orange). I turned on the ftp server and it took the file via, went to 100% completing the BT transfer but again, when I try and play it I get the exact same message. This file was also mp4. This file works on his Desire (it is a 26 minute cartoon called the Boondocks) but not mine. I cannot understand the inconsistency as you would think that the same would apply to both mine and his.My last phone was a n95 and I am not someone who regularly takes an interest in new gadgets coming out, making this big jump (only ever used Nokia's and Ericson's before the desire). However, I have had a new phone (always top of the range through upgrade) every 12/18 months since 2001 so I am very much used to adjusting to new phones, albeit not the 'smartphone' generation. So I am not a technophobe but nor am I a tech geek. But I cannot for the life of me work this one out.