I bought a Gateway laptop with W8 with 4g  of ram and a 500 g hard drive. It has an I3 processor. I was looking at the PROCESSES that were running. There is a column that has disk usage. It showed I had 100% usage. How can the computer be using 100% of a 500 g hard drive?The 100% is a measure of your RAM being used.The "Disk" column is a measure of how much disk activity is being caused by each process, not space usage.  Every storage device can only transfer data so fast, if your usage column is showing 100% then the drive is transferring data as fast as it can, so it'll most likely perform pretty slowly until that's finished.
Can you see what process is causing this utilisation?Avast antivirus was CONSUMING the most usage. The computer was pretty slow. I bought this computer recently from a pawn shop for a friend of mine. After looking at it Avast was a trial version. When I switched to the free version the disk usage went down considerably. I would imagine that Avast would've been SCANNING files, but continuous 100% disk usage isn't normal unless it's running a scheduled scan.  If it's dropped down now, I'd just keep an eye on it.  I'm not sure how Avast compares to other ANTIVIRUSES these days, I use Microsoft Security Essentials (or Windows Defender as it's now included in Windows 10) and haven't had any reason to try another in the last few years.