My printer is 600 DPI so a line that wide would need a magnifying glass to pick up the jaggies.
Maybe your canvas pixel size needs to be a lot more?
I have used Paint Shop Pro 4.1 shareware for many years.
I do simple projects: Right now I am drawing a city roadmap.
When I draw a near-horizontal line two or three pixels wide it comes out jagged, and PSP 4.1's smoothing tools and filters either blur the line too much, or not enough. I think I need a later version with better tools - but not so advanced that it will take me weeks to learn to use it.
Keeping in mind that I am running XP, any suggestions for what
program to get?