Many people have the habit of splitting files for easy transport or due to space constraints. Hence, the files are split and mostly RAR archives are used. Always use WINRAR for these files.
In order to join the contents of these files download and put all the part [all the part rar files] files in the same directory and then take the part1 file ,open it and extract it’s contents using winrar. Automatically the .iso contents of the other .rar archives will be extracted and combined. If one or more of the part rar files are missing winrar will ask for the location of these part files.