Is it safe to delete this file?

I'll reiterate a very common response: there's no way to know what a ".dat" file is without knowing the application that created it. The file extension ".dat" is often used to represent "data", and as a result is used by many, many applications. There's no way to know what the file contains, or what to do with it without which application created it.

So, how do you determine if a file - any file - matters?

Well, my first suggestion is perhaps the most important one:

Before you do anything, backup the files.

Copy them to CDs, copy them to another computer, just do something, anything, such that if you discover that you really wanted that file after all, you can get it back. You may find that the file was an important part of some application that you rely on daily. Or worse, an application that you only use once a month or so. If you simply delete the file, it's gone without reasonable hope of recovery. If you've backed it up somehow, you'll not only be able to remove from your system, but you'll be able to put it back if you discover that was a mistake.

My next steps are fairly simple:

First, rename the file or files. Use your computer for a while. Reboot. If an error occurs related to the now missing original file name, you now know what the file was about, and can decide appropriately based on the error you get. If you discover no errors, then you're ready to move on to the next step.

Next, delete file or files. (You did back them up, right? Right?). Same exercise ... use your computer for a while. Reboot. Use it for different things, exercise some of the applications you have on the machine. Once again, if an error occurs related to the now missing file, you now know what the file was about, and can decide appropriately based on the error you get. If you discover no errors, then you're done. Sort of.

Last, remember that you deleted the file and where you put the backup. This is probably the hardest part for me :-). The reality is that our simple tests above may not actually have fired up the application or scenario that needed the file. You may not find out until some lengthy time down the road that ... whoops! ... the file you deleted some months ago turned out to be part of an application you haven't touched in ages. Hence, again, the importance of the backup.

What if I can't delete or rename the file? If it's because the file's in use, that's great! Why? It tells you that the file matters, and you can then use a tool such as Process Explorer to figure out who's using it. Once you figure that out, you can once again decide the right course of action based on knowing what application is accessing the file.