Over the past few weeks the company I work for has been having issues on their server wherein a file gets locked by something, and can't be unlocked. These files are almost always files that are being actively accessed over Windows file sharing. I've downloaded the popular Unlocker program, but that is unable to find any lock on the file. openfiles.exe (or System Tools > Shared Folders > Open Files) does not show the file as being open by anything. Yet I'm unable to open it in anything, including notepad - the system gives me a permission denied error even when accessing the file as the local system administrator. The problem always goes away when the server is rebooted.
To make matters worse, I've also been unable to reproduce this error at all. It's frequency is anywhere from daily to a week of working perfectly fine. When the problem does happen, I have very little time to investigate it, because nine times out of ten the problem makes our production systems completely nonfunctional, and management wants the server rebooted.
The problem is also not limited to a single application. The majority of the file sharing traffic is in our production management software, Globalshop, so the problem usually manifests itself somewhere in there, however the most recent occurrence was in a Microsoft Access database completely separate from anything Globalshop.
Has anyone seen anything like this? Any idea what could cause it? Let me know if there's any more information you need.
We're currently running Windows Server 2003 Standard Edition.
 I've tried checking permissions, but I'm disallowed access to those as well on the offending file. I can't open, move, delete, rename or even copy the file.
[edit2] I've managed to isolate two offending files. As long as no other production-interfering problems crop up, I've been authorized by management to keep the server online for the rest of the day to test them.
[edit3] Brettski, in comments, suggested that I look at the file permissions. I've taken a screenshot of the properties window of one of the offending files. As you can see, there are no permissions available. http://i43.tinypic.com/24xgpe8.png (This is opened as the system administrator with full access rights.)