Yea you shouldn't delete that file. As pelvis said it is a system file. The dat.log file changes size because it is a log file. It logs information then when it gets too big or at a time specified by your system it empties itself. The reason it reads 1 Kb and not 0 Kb is most likely because it stores some residual information when it empties itself. You can also open it in any text editor (Notepad, Wordpad, Word) and see the contents, if you know what you're looking at. Also a bit of advice, if you want to know what a file is simply enter the filename including extension into Google and almost 100% of the time it will be in the first or second result. And I would recommend you turn "Hiding files and folders" and "Hiding system files" so you don't accidentally delete something very important.