No doubt at some point you have tried to delete a file (or folder) and have received the following error:
This means that an application is currently using this file and you cannot delete it, this shouldn't normally happen but may occur if a network is using a shared file or a program hasn't closed a file properly.
Usually, a restart will allow you to delete the file/folder safely - but if you don't want to restart or the problem persists then you need to download a 3rd party tool.
There is a great freeware tool called Unlocker that lets you delete any file/folder in use by right clicking on it and selecting the unlocker option. This stops applications from accessing the file and deletes the "in use" file:
Be sure that you do not do this to any system critical files, as it is possible to stop Vista from loading if you use this application improperly.