50 GB is very small for an OS plus programs. Remember that Win 7 does not just install what you tell it to install to your C: drive. It creates a swap file equal to the size of "virtual memory" (pagefile.sys), usually a hibernation (hyberfile.sys) file equal to a three quarters or more of the size of the computer's actual memory and a huge amount of stuff in the area where restore points ( including previous versions of files if enabled),  and system images are kept. You can only see those files from windows if you set explorer to show hidden and system files. A lot of that stuff can be moved to a bigger drive or partition with some effort. There are also a lot of files stored by default that enable you to roll back to an earlier version of Windows or to undo the installation of a windows update or hotfix. A lot of that can be deleted or moved to a backup drive. MS assumes you have one infinitely large drive on your computer and puts everything on that one drive by default.
 
 

Windows Updates will eat up space over time.  You could easily have 5 GB of update files.  Microsoft did a change a few months ago that now allows the updates to be cleaned up using the Disk Cleanup utility mentioned above.  Make sure you reboot after this as it needs to finish cleaning up the files and it won't do it until after you reboot.
 
Jim

slartabartfast
MS assumes you have one infinitely large drive on your computer and puts everything on that one drive by default.
 

As do some other guilty parties...  Not to name names... Well, maybe NI.
 

Well Ni software installation is pretty reasonable nowadays apart from all the useless prompting. With Komplete I managed to get it to put samples and VSTs to where I wanted it to be. It still insists on installing 32 bit plugins even when I only need 64 bit, doesn't like antivirus (as it mounts ISO images all the time) and won't install off network drives (pretty shabby really).
 
 

mixmkr
some may shoot me for this...but go to "filehippo.com" and download "CCleaner" and run it.  It will show you a large amount of stuff that can be eliminated, and you'll have the option to keep what files you want as well.

This is excellent advice. CCleaner gets rid of all kinds of unnecessary gunge. I run it every few weeks.

Hello and thanks again for all your suggestions!
 
Here are my replies and further info:
 

• The total size of the physical hard drive in my laptop is 320GB (biggest size that was available to this laptop).

• As I said in my original post, the drive is partitioned into three drives: C (OS & application), D (library contents), and E (everything I download, create and save). (mudgel, you’re right, partitioning was probably a bad idea. I wonder if all laptop users are using an external hard drive for library contents, projects, presets, etc. I only use my external drive for backup purpose. I may need to reconsider my system...)

• When I said the sizes don’t match (32,308,194,774 bytes vs. 50,685,140,992 bytes), the sizes of the folders I calculated are “Size on disk,” not just the “Size” (if the “actual file size” mudgel mentioned refers to the latter).

• Service Pack 1 is already installed. (I should update my sig.....oh, actually, it already says SP1)

• The recycle bin and temporary internet files are included in Disk Cleanup, so, yes, I emptied them already

• My “Temp” folder in User/App Data/Local/Temp says only 428KB (“Size on disk”). This isn’t included in Disk Cleanup’s “Temporary files,” is it? Because “Temporary files” says only 13.8KB.

• My default “Download” folder in C drive is empty, since I had changed the default download path in my browser preferences (Chrome). Any other “swap” files I should check? Should I just search for “hyberfile.sys” and move it to a backup drive?

• The folder size of my c:\users\your_name is currently only 108KB (“Size on disk”). Most of the folders in there (music, pictures, videos etc.) have nothing in them.

• Thank you very much Bob for the procedures to identify recently added files, etc. I’ve been wanting to know it (I know, I should google it but I’m lazy doing it when it comes to PC things). I’ll try it later.

• How do I decrease the percentage used for restore files? Sorry for my ignorance…

• I googled WINSXS now and checked the folder, and….yes, it’s huge (8.49GB) and I’m not supposed to delete its content, am I? This is probably the biggest culprit and will fill up my C drive eventually.

• I’ll check out TreeSize. Thanks for the info.

• Is the recent change for Disk Cleanup to have the ability to delete Windows Update files only made to Windows 8? If it was made to Win 7 as well, which item in the Disk Cleanup list is it? I don’t see anything with a big size in the list anyway…

 

At the moment, I prefer not to do another OS re-install (I already did it twice this year and they were a hard and time-consuming job). So, I’m thinking of either using CCleaner or doing the monthly cleanup which emwhy suggested (I have no idea which is better). I did a full backup last night so I can try anything now, and I probably have several days before my C drive becomes full. I’ll try my best.
 

Anyway, thank you all again, and I’d appreciate any further info. As you already know, I don’t know much about PC. I love music but hate computers. They keep changing and I can’t follow them. My previous PC was a desktop with only a 16GB hard drive and I was making music fine with Sonar 2 with mostly hardware synths. Times have changed. I feel like smashing my laptop and hit the trail with just an acoustic guitar. But I know I can’t…
 
Thanks

The Disk Cleanup update is in Win 7 too, but it's a bit weird to use. You have to run disk cleanup and go through the scan, and the Windows Update files initially won't show in the list. If you then click the button marked "Clean Up System Files" (requires admin privileges) it will do the scan again and add them to the list.

http://www.howtogeek.com/howto/windows-vista/resize-a-partition-for-free-in-windows-vista/

John T
The Disk Cleanup update is in Win 7 too, but it's a bit weird to use. You have to run disk cleanup and go through the scan, and the Windows Update files initially won't show in the list. If you then click the button marked "Clean Up System Files" (requires admin privileges) it will do the scan again and add them to the list.

I use this AND cc-Cleaner.

Yep, me too. About once a month, I do a general house tidy with both of those things. Once you get to know the settings, you can leave them as they are and just nuke it. Takes me two minutes these days.