so u got a 20gb hdd and only 150mb of free space left. yet u know the demo is 1.77GB, so why download the demo when u know you do not have enough space left on ur hdd?
could also just look in My Computer and see how full the drive is to know how much is used
anyway, you probably don't have enough space due to the fact that when its being downloaded, the file is being stored in a temporary downloads folder. Then, once the download finishes, the file is moved over to the location you chose to put the file (the 'transfering to (location)'. You might have had just about enough space to download it, but the problem is that during the file move, the old file can't be removed until the new one is finished being created. if you didn't have enough space for the new file to be created before the old one was deleted, well, theres your problem.
and...geeze...20gb hard drive.
you can get a 500GB for around $110 from newegg.