One method you can try is disabling explorer incase that's the cause for the problem...
Start an Administrative Command Prompt... Leave it open
Start Task Manager, Close Explorer.exe Attachment 68470
Open the command prompt.
Change to the appropriate directory: cd "%USERPROFILE%\Downloads"
Delete the file: del filename.mkv
- ENTER Attachment 68471 Oh yeah, almost forgot. Re-initialize explorer.exe by going back to Task Manager and clicking the File Menu > Add New Task (Run) and type in explorer.exe Attachment 68476 In response to your task manager screenshot:
Holy Mother! How many extensions/add-ons do you have on firefox? It shouldn't be using 285K of memory! The screen shot below is firefox w/ 4 tabs open & a custom theme with 6 add-ons.