Photoshop scratch disk

One other thing to consider.  You mentioned you deleted a lot of files already.  They are not really gone, just ready to be overwritten.  Look into defragging your HD.  There are 3rd party apps that can do it for you.  You could partition your HD.  That would require reformatting.  Are you able to install another drive in your computer?

If you only have your internal drive and its close to full, you should strongly consider adding an external drive. That way you can backup important files and free up space on your internal drive.

How much free space is there on your system drive?

You can change your scratch disk preferences only if you have another physical disk installed. It's possible to get it to work with external drives, but that will probably be slow and is not an ideal solution.

Photoshop moves massive amounts of data around, and it requires a lot of free space for temporary working files. This is the scratch disk. You really should have 50 - 100GB available at least.

If you really want to clean out your system drive, you should look under your user account. That's where the junk tends to accumulate, and normally where you can free up the most space.