Please see the link and comment. Staff on that forum is not nearly as helpful as you nice people on Avast forum. 
http://forum.utorrent.com/viewtopic.php?id=68336
Thank you!
Could someone please tell me if the described file behavior is normal/not dangerous? Thank you in advance!
I wish I could help you but I can not. I hope someone will offer an answer for you soon.
Hi :
IF you mess around with P2P programs, like uTorrent, you are asking for problems.
Hi all…
I fully agree, in more ways than one. 
God bless
Torrents software is bad news.I would not touch it.I agree with Spiritsongs and ardvark on this one.
first let me agree with them. turrents can be problems, just like any other file sharing program.
You said that you tried to laod the 350MB avi file and it load as 650MB. could it possibly be that the program that you are using was trying to convert the AVI file into a VCD? that is about the only thing i could think of and that would make it the size of a CD which is 650MB in size by some programs…
I think moogly and Switeck on the forum were right: it’s just a ghost in the machine, an artefact of the way uTorrent allocates and reuses disc space.
Signed: evil torrent user.
No need to worry.
It is just a ghost. It will go away on its own when some other data is written over it.
;D
Thank you all!