Error: Can't move mkv files to another folder

Hello,

I have some movie files that I moved recently to another folder. When I tried playing them today they wouldn't start. When I tried to move or delete the files I got the following message:

Cannot move (insert file name): Access is denied
Make sure the disk is not full or write-protected and that the file is not currently not in use.

What can I do to get these files back or at the very least delete them?

I'm usinf XP Pro and I am the only user of the computer, I don't believe there is any virus or malware issues either. The HD has 350GB free so the disk drive is definitely not full.