Disk space problem, do I extend partition?

Do you have backups? You may well be facing a clean install as an in-place upgrade repair is not possible with your PC as is. (You need more space, and you might not be able to start it based on the non-responses).

First check your disk. You can try a portable program (run it from a flash drive).
Download Portable HD Tune 2.55

Use the Health tab, then the Error scan tab.

Post the results.

Yes you can save that space- if you're able to use your PC at all, which it doesn't sound as if you can.

Now's the time to create backups. It's never too soon. What will you do when your disk fails?

I don't think you'll be happy with just using the command prompt. I remember the days of MS-DOS.

Then please run the disk test. Knowing nothing about your machine, have to start from basics as you have no way to recover presently, and too many things show problems.

gecksy said:

Ran wiztree and seems my iPhone backup is taking up a large amount of space on the C:

Can I change the drive where the backup and other things are saved to?

I don't have an iPhone, but apparently the answer is yes, you can...

fonepaw.com | How to Change iTunes Backup Location in Windows 10/8/7

...or you could just delete them.

tekrevue.com | How to Delete iPhone Backups in iTunes for Mac and PC

Excellent..

Now please create a new user for test purposes and see if you see the same problems.

This is a key test, as it distinguishes between things related to your user profile, and basic Windows corruption.

dalchina said:

Excellent..

Now please create a new user for test purposes and see if you see the same problems.

This is a key test, as it distinguishes between things related to your user profile, and basic Windows corruption.

Issue remains on the other user, I even had to create it via command prompt

Thanks. Please make sure you have a backup of all key data.

Please run
chkdsk c: /F
from an admin command prompt to check and repair the file system. Your PC will restart.

What is your Windows build? If unsure, please try
Windows key + R, winver
or command prompt, winver

If not 1803, please post back and do not proceed.

If 1803: then do an in-place upgrade repair install which keeps all programs and data and most settings.
Repair Install Windows 10 with an In-place Upgrade | Windows 10 Tutorials