I have my own web site with an automatic 10Gig of space from my hosting company, so I have never used a service like dropbox... I suggest Googling to find out how to use that, or another file sharing service
As for your problem with some AVI files...
AVI is a wrapper, what is inside YOUR wrapper - Exactly what is INSIDE the video you are editing, that does not work?
Codec & Format information, with 2 links inside for you to read http://forums.adobe.com/thread/1270588
Report back with the codec details of your file, use the programs below... A screen shot works well to SHOW people what you are doing
https://forums.adobe.com/thread/1070933 for screen shot instructions
Free programs to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download
Does your video use a VARIABLE Frame Rate or a Constant Frame Rate?