For beginers it often comes down to access/cost. If you were to buy a copy of ESRI's ArcView it would run you about 2k, if my memory serves me well.
If you have a cash flow problem you can hypothetically load a peer to peer service on your PC and accidentally download popular GIS apps on to your PC. But I've just heard about these kinds of things, I would never endorse them.
www.autodesk.com => You'll find AutoCAD based GIS software burried somewhere on this site.
www.esri.com => This site will try to sell you every type of GIS product you can think of... So don't get overwhelmed by al of there flashy extensions... keep focused on their core products (ArcView, ArcEditor, ArcInfo)