This problem occurs when windows is trying to show you a preview picture of your video, whilst you are trying to delete the file. So then you get a message telling you the file is still in use and the delete operation fails. Solution? Stop windows from showing you thumbnail images of your videos.
Start -> Run -> regedit
Edit -> Find
In the search box type this:
Tick the box titled ‘Keys’ and untick the others. It should find the key for ‘AVI Properties handler.’ Right click the key from the left window pane (with the folder tree view) and click export. Choose a file name to export the key to, and click OK. Now you can delete the key you just exported. If anything goes wrong, you can double click the .reg file you just created to restore the key.
You can also delete the following keys:
Video Media Properties Handler
Video thumbnail extractor