Did this folder/drive (your document/video folder) exist before you updated?  
I had the same problem when I installed Vista as a 2nd os.  You might have
better luck if you turn off User Account Control, but you also might need to
get those files into a new drive or folder created from within Vista.  Are
you dual booting?  If so you could create the drive or folder in Vista, then
boot to your other os and move the files into it (assuming Vista also gives
you a problem with moving or copying the files.).  First try to copy your old
files from Vista.  Good luck.  I think Vista is a bit too aggressive in its
security, especially for home users.

Hello,

The videos folder is no longer inside the Documents folder. It is now
located directly inside your user folder (it is on the same level that
Documents is).

What you see that is called "Videos" inside the Documents folder is not
really a folder. It is called a junction, and cannot be accessed. It's
purpose is to point programs that try to access it to the new location of
that folder, which is up one level under your user directory.

Instead of:

Username
- My Documents
-- My Pictures
-- My Music
-- My Videos
-- My Pictures

In Windows Vista, you have:

Username
- Documents
- Music
- Videos
- Pictures

- JB

Vista Support FAQ
http://www.jimmah.com/vista/

Try copying or saving files you definately want to be used in different
Windows versions on a multiboot/multiuser system to the Vista Public folder
likely path is
(your sysdrive letter such as K: in XP but may show as C: in Vista)  
K:\Users\Public\
This has been working for me on my quadboot system.
Accessing a file shared folder over the network is also easier.
Better to keep UAC since it is the advance in security with Vista.

Incorrectly changing the permissions or ownership of  a folder or file may
result in access issues from one OS or the other.
Hopefully priviledges will be easier and clearer in RC1

MG