At some point in your computing life, it's very likely that someone who believes he or she is more technically minded than yourself has told you to clean up your PC or Mac by deleting unnecessary files. On the one hand, you'd love to take the advice, because you know it could potentially make your computer faster. On the other hand, if you delete the wrong files, you might break something, like accidentally disabling a vital component of your operating system or rendering some expensive software useless. And who wants to fix that mess?
If you're confused by such issues, in this article, I'll help you figure out what you can delete on your own, what you should delete but not by yourself (there are free tools, and they take mere minutes to install and deploy), and which files you should leave well alone.
Files you can trash
Personal files. Any file you created is most likely safe to trash when you no longer need it. These include documents, photos, and even music files. In many cases, you'll want to archive the file first; in other words, make a copy of it that you'll save to a back-up location, like a disc, external hard drive, or online cloud storage service.
Note that cloud storage and cloud-based file-syncing programs do not work the same way. If you have a file "synced" to a file-syncing service such as Dropbox or SugarSync and you delete it, the file will also be deleted from the cloud. (One workaround is to upload the files you want to save via the web interface for the file-syncing program, which makes them safe to delete, but that solution only works when you have a handful of files to upload).
For more advice on deciding how to clean up your personal files, see our article entitled Staying organised: File deletion.
Zipped or Stuffed files (post-extraction). When I download a Zipped or Stuffed file, I like to extract all the compressed files from it to make them easier to manipulate, and then trash the original file, which often ends with the extension ZIP, 7Z, SIT, or RAR (although there are many more compressed file formats). You can trash it after extraction because you'll now have a separate copy of all the included files in the resulting folder.
Conversely, if I Zip, Stuff, or Archive (the included tool in Mac OS X) a set of files that I want to keep on my machine but might not be using any time soon, I'll trash the original loose files and keep only the compressed file. Tip: Before trashing the original files, always test a compressed file by trying to open one or two of the contained files at random to make sure the compression worked properly and did not corrupt your files.
Icons that look like mounted drives but aren't. Sometimes after you install software, especially on a Mac, you'll see a new mounted drive hanging around on your desktop with the name of the recently installed program.
If the program works (go ahead and launch it from the Applications list), you can trash the icon or hit the eject button next to it in the Finder window. If the program doesn't launch properly, delete all the associated files and try reinstalling it from scratch.
Executable files. Similar to the previous point, if you download a program, install it, and can launch it with no problems, you can trash that executable file – although depending on the program, you may not want to.
For free downloads, go ahead and toss the EXE files. For paid software, if you don't have a disc for the program, just make a copy of the files somewhere safe, like a disc, USB key, or similar.
Don't touch these files
Hidden files. Both the Windows operating system and Mac intentionally hides files from you. Any time you come across a message about hidden files, take it as a big red stop sign (unless you are an advanced or ambitious user, which I'll touch on momentarily).
Most computer users should not delete hidden files. The fact that they are hidden is a built-in safety net to prevent users from screwing around with files they ought not touch. Don't worry about what they are. Ignorance can be bliss in this case. If you are an advanced user (or are in the stages of becoming one), there are times when you might want to delete hidden files because you're sure you don't need them and they are simply eating up space on your machine.
Anything you can't identify. If you don't know what a file or folder is, leave it alone. In the image below, you can see a column for Kind of file, and a bunch of them are "Workflows" and "Script bundles." If you don't know what these things are, don't touch them. If you do know what these files are, or are willing to do some research on them (i.e. Google 'em), there may be some you might delete (and in which case, see the note in the previous paragraph regarding advanced and ambitious users).
It's true that sometimes you will want to delete files that you can't personally identify. It's best to assign that chore to someone – or in this case, something – that knows the difference between needed and unneeded files, like temporary Internet files for example.
I personally use a tuneup utility called CCleaner for Mac (it's free, and also available for Windows). Every few weeks or so, I open the app and push a button, and the program does the rest. You can also set this cleaner or a similar one to automatically run scheduled clean-ups if you prefer.