Cannot copy <filename>: Access is denied

This is a problem that cropped up suddenly without me being aware of
installing or changing any settings on my computer or laptop.

On Computer I have XP home
On laptop I have XP Pro with simple file sharing enabled

I have two extra hard drives in my pc, J: and K:

The problem is I cannot copy files from my pc to my laptop over my wireless
network, I get the error Cannot copy <filename>: Access is denied. - Make
sure the disk is not full or write protected and that the file is not
currently in use.

This is where it gets confusing:

From my pc I cannot copy anything on to my laptop's share from my K:
although I have been doing it for a long time, but I can copy anything from
my J: still.

From my laptop I cannot copy anything recent from a particular folder from
my pc to my laptop, but I can with some older files in that same folder
(seems to be more of a problem with avi files). I can copy from any other
folder on my pc's share as long as i use my laptop to do it. This
inconsistancy is whats confusing things.

First of all obvious thing out of way, my disk is not full, read-only, or
write protected. The files I am trying to copy are also definitely not read
only and I have used software to ensure there is no other program locking the
files.

I have done nothing regarding changing permissions on files so I can't see
why that will be the reason either.

Lastly another recent problem which may be unrelated I'm just mentioning it
in case it helps, I can no longer save Word documents directly to my USB pen
drive, it says it cannot create the file. But I can save them to my local
disk then move them to the drive. This is not the problem I need answering
at moment though, I'm just saying in case it helps with the other one.

Thanks in advance for any help

--
May the monkeys be with you