virus?
delete in safe mode?
or..........
security/permissions
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One of the possible reasons you can't delete a file (or directory) is that you don't have the rights to do so. It is possible to have Create and Modify rights without having Delete rights. Another possibility is that the file itself (or, more probably, the extended attributes of the file-its metadata) is corrupt. I once had a hard drive that did this on a regular basis. In fact, it was a bug with NT 3.51 with a really huge number of files. The only way to cure that particular problem was to run chkdsk /f on the drive and then reboot the computer.
A real possibility with deleting a directory is that you have another program accessing that directory (or one of its subdirectories). Check for other copies of Windows Explorer (or My Computer or My Network Places or even the command prompt) running and check to see where their current directories are pointing. Also look at any application programs that are running and where their open files are located. With programs like Notepad, it really doesn't matter. You can open a file with notepad and then go to Windows Explorer and delete the file you just opened. Notepad actually opens the file, copies all the data, and then closes that file. However, if you used Notepad to open the file MyFile.txt in D:\MyDir, even though NT allows you to delete the file immediately, you cannot delete the directory while that copy of Notepad is open, because Notepad still has a pointer to that directory.
A common reason for not being able to delete or move a file or directory is due to a program crash. If Word crashes, for instance, you might not be able to delete its temporary files. Most of the time you can cure this problem by going to the Task Manager and looking under Processes. In this case, you would be looking for an instance of Winword that you could then end from Task Manager.
You can also fail to delete a file (or the directory that file is in) if a file is being accessed remotely, either via DCOM or a shared directory.
Some applications have their own little quirks. Visual Basic has a number of these, including the inability to compile an ActiveX component (in some cases) until you have exited VB, then reopened it again. This normally occurs after you have run the application from design time and you get a "Permission Denied" when you try to compile it. Or you can use the Open command to open a file from VB and then stop the program during design time before closing the file. Normally, you can get back to square one by closing VB and opening it again.