EDIT: in task manager, my disk is at 100%. Is that abnormal? There's room enough in it though.
Task Mgr Disk utilization of 100% simply means that the disk is constantly reading/writing. That has NOTHING to do with free disk space
Could 100% disk utilization be an issue? ... maybe.. depends
The issue is disk I/O latency and queue depth. If tasks/processes are waiting for the HDD, then yes, that can cause issues. Using Resource Monitor can provide more details, but to get meaningful info you'll need to learn about what that monitor is displaying and what it means
If I recall correctly, you also only have 4GB of RAM which is on the low end, and may be cause for virtual memory disk swapping (ie extra disk I/O)
I had an even older CPU until recently (power supply went up in smoke after 10+ years). But I didn't use that for the computationally intensive tasks of real-time video encoding. One thing I did long ago was switch to SSD as they are SO much faster than HDDs, especially for OS/App drive. But on a PC of such an age as yours (9 CPU generations old), assuming that is a HDD in your system, putting money into a new SATA SSD would be of questionable value in my mind. Now, I strongly recommend SSDs, with proper care (ie always make sure 10%+ spare space) as that will amke typical overall system performance (ie Office, emails, etc) improve greatly. But for photo/video editing, real-time video encoding, or other Apps designed for current processors, that old of a CPU will almost always struggle, at best. Up to you to balance your time [to optimize the setup you have] vs available money to upgrade