That's a blast from the past. "PC Paintbrush" was the name of a paint program published by ZSoft, Inc. It was one of the first paint programs for IBM PCs around. ZSoft got bought by WordStar, who got bought by an outfit called SoftKey. The SoftKey domain name now takes me to a page for The Learning Company, so perhaps they got bought once again. At any rate, it seems that Paintbrush is no longer published.
On the bright side, there are a number of nice graphics programs out there. For freehand sketching, I'm fond of Metacreation's Painter. (Used to be called Fractal Design Painter.) You can get a stripped down demo of the previous version
from their web site. It has lots of interesting brushes and surfaces that attempt to mimic natural media.
Adobe Photoshop is a nice tool, but it's a little more oriented to photo manipulation and editing, whereas Painter is meant for direct creation. Paint Shop Pro, from Jasc Software, is a cheaper alternative to Photoshop. I also ran across a demo for "Chaos Fx", which seems to sort of split the difference between Painter and Photoshop. It seems a little rough around the edges, but it's relatively cheap.