I’m sure most of you have encountered this type of error, at one time or another, while using Windows:

It appears when you try to delete a file or folder that is currently in use by an application. To be completely accurate, the file needs to be in use and locked by the application to prevent external edits – the lock is likely the reason we see the error message. Sometimes it is really obvious which application has a locked handle on the file – for example, if it is complaining about a Word document – perhaps the document is still open in Word. Personally, I consider scenarios like that acceptable because they can be solved easily; unfortunately, all too often I get into trickier situations where I can’t figure out which application is holding onto my file. In those cases, I used to just reboot, and that would usually solve the problem. Yet, there were still cases where rebooting didn’t even do the trick.
Eventually, I was tired of struggling in those moments, and began searching for an application that could help – that’s when I found Unlocker. It allows you to remove locks from individual files via an “Unlocker” option within the Windows context menu. Definitely a nice feature… but listen to this – they took it a step further via a system tray program called Unlocker Assistant. If you encounter the “Access is denied” error while Assistant is loaded, Unlocker will automatically pop up a few seconds after you click “OK”, and it will show you the culprits:

From their interface, you have the option of either killing the process, or unlocking the files themselves – effectively breaking the hold the application has on the file without having to terminate it. Very handy. You can download Unlocker here.
If you’re more inclined to trust Microsoft-sanctioned applications, grab Process Explorer – a Sysinternals app. I didn’t try it for this purpose specifically, but from what I’ve read, it isn’t as simple as Unlocker when it comes to removing the “Access is Denied” error specifically. It serves multiple purposes, whereas Unlocker is specialized.