There are times when I'm renaming or deleting a file or folder in Explorer, I'll get an error message saying that the file/folder is being used by another person or program, hence I cannot rename/delete the file/folder. But after a few seconds, I can rename/delete it without the error message.

Is this caused by a virus or spyware/malware? Or is someone sneaking into my computer via the internet? I'm using a router and I have firewall installed.