As soon as they realize Linux is also a Personal Computer
More likely: its is because software developers put For PC on the side of software boxes for Windows, and For Mac on the side of software boxes for mac.
So it may actually be:
PC = Windows
Personal Computer = Computer for personal use
I believe you mean the program runs out of room to install in.
Un-install some some programs you don't use. Delete any large files that you don't need. And if you still don't have enough room, get a bigger hard drive.