Corrupted folder contents.Virus or what?

A friend of mine was showing me latest video files he got from his usb flash
drive on his system when we discovered that 2 folders on the flash drive are
messed up & the contents of their video files are replaced by lots of files
with unreadable non sense names, moreover, those folders were un-deletable
(logical, huh?).
He removed the good files & folders to his system, formatted the flash drive
twice, copied some clips & movies for me on the same flash that I took home.
When I opened it I found that 1 of the folders I copied to the flash its
contents was replaced with the same messed up files. I tried to save the good
files (dunno good choice or bad, but I needed some of these files badly) to
my system then reformatted the flash drive & disconnected it.
I found that some of the files I copied were unplayable so I deleted the
(shift-delete) at once.
I use Win XP SP2 with Eset Smart Security 3.0.650 updated daily, Ad-Aware
2007 with Ad-Watch on, updated daily too & Super Antispyware updated daily,
All with real time protection activated, & NONE gave me any warning or
threats.
Is this a virus or trojan...etc? if yes then which 1& how to get rid of it?
If no then what's going on?
I scanned my system quickly & found nothing.
Is my system infected or in danger?
What should I do?
Thanks