Why are some of my file names blue?

First let's show what the question is referring to:

This is a partial listing of the files in my "C:\Windows" folder. You can see that some of the folders are displayed in blue, while others are shown in the more common black.

In the NTFS file system, files and folders can be marked as "compressed". This means that when the file is written to disk it is compressed to take up less actual disk space. Files added to a folder marked as compressed are similarly compressed.

Not all files can be compressed, or rather, not all files get smaller when you compress them. I know that seems backwards, but if a file is already compressed, then attempting to compress it a second time can actually make it a little larger instead.

Many files these days are already compressed. The best examples are most audio files like mp3 files, most picture files like the jpg files that come off of your digital camera, and most video files that you might get from your video camera or shows that you download.

That's one reason why file compression is typically off by default; you don't want to just blindly start compressing everything, because not everything should be compressed.

Another reason is that compression and decompression when the file is written or read takes some extra CPU time. These days it's not really a lot, but it can add up if the file is accessed a lot.

So what files are good candidates for compression? Archives of things that aren't already compressed and that you don't access a lot are good candidates.

But then again, with the size of hard disks being what they are theses days, I rarely compress files, as there's simply no need to add yet another something to what happens when reading and writing files. I only compress files when a disk is running low on space and it's impractical for me to get more space by other means.

Let's look at how to adjust that Windows Explorer setting, and then also how to actually compress files or folders.

In Windows Explorer click on Tools, Folder Options and then the View Tab. In the Advanced settings list scroll down until you see Show encrypted or compressed NTFS files in color:

Make sure that's checked, or not, as you desire. (In case you're wondering, encrypted files display in green when this option is selected, and compressed files in blue.)

Now, as for compressing files or folders, simply right-click on the file or folder you want to compress, click on Properties, click on the Advanced button, and you should see something like this:

Make sure that "Compress contents to save disk space" is checked as appropriate and OK your way back out, and Windows will compress the file or folder.

(Since you might be tempted, I recommend against Windows built-in file encryption unless you truly understand the ramifications. In a nutshell, it's tied to your Windows login, and if you ever lose that you will lose the files or folders that you've encrypted - permanently. If you need encryption, I recommend a solution like TrueCrypt instead.)