Few days ago I was scanning a memory card with Avast and PC got shut down. Now there is file of 4GB in C:\ProgramData\AVAST Software\Avast and I need permission from SYSTEM to delete it. I tried to take permission from administrator account but didnt work. What should I do to delete this file?
If the OS got shutdown during a critical I/O operation to the memory card, it could have creamed something in the File System Tables on the card. This can make removal of the file very difficult. Windows can be really porky about this. If neither of the previous suggestions help, you may need to copy all the readable data to another media and then reformat the memory card. However, if you know someone with a different OS (or have access to a computer running a different OS), try deleting the file on a different OS (eg, linux, unix, Mac, sun, etc), but you should still save whatever you can on a different media first. I believe there are also several "Repair" programs out there, but they may not work anyway and on Windows, they will cost you. If you're not losing anything you care about, a reformat is usually the best/easiest solution.
If you know what you are doing, then open a cmd window as System and run delete file from there. So, in a admin cmd window, enter:
at 12:34 /interactive cmd.exe
where 12:34 is, say, one minute ahead of your current time.