1) You may use Winrar or IZArc to decompress it. As IZArc is freeware, so, it is recommended. http://www.izarc.org/download/IZArc35.exe
2) In IZArc, you could select abc.part1.rar or abc.part2.rar file, then, you could decompress all the files inside the .rar file.
1) download rar software from www.download.com
2) install the rar software
3) double click the 2 files
3) extract the files into specific folder
4) finished