Need advice: what open-source GIS do you recommend for a novice, and why?

I’ve been using Bing maps to make distance and area measurements, and have saved a large collection of kml files. I need to begin using a true GIS to enable location based queries. I took a GIS course years ago that used ESRI’s ArcMap. Although that was very frustrating, I find basic GIS concepts are pretty straightforward. Still, I will need to teach myself, and I have a very limited budget. So, I’m looking for a free GIS package that would be relatively easy for a novice to learn, allow for data conversion from the kml collection, and hopefully have datasets available for base maps (streets, political boundaries, aerial images, etc.). And I’m running XP. Any suggestions on which direction I should proceed? Thank you everyone for your help!