Probably WinRaR. You could also try the freeware 7-Zip. Usually .rar files end in XX.r00, XX.r01, etc. The 'Z' might refer to 7-Zip, though. WinRaR is a bit easier to use with multiple files. The programs should rejoin them all into a single file.
WinRar: http://www.rarlab.com/
7-Zip: http://www.7-zip.org/