The best tool for this is IZARC... With .rar files you will have one of two naming conventions...
movie.rar movie01.rar movie02.rar etc. in this case go for the one with no number in it and izarc will reassemble the movie for you.
movie.rar movie.001 movie. 002 etc. in this case fo for the one with the rar extension and voila ... playable movie.
IZArc does take a little bit of setting up but it's nothing "joe average" can't do in 5 minutes or less.
You can just download the trial version of winrar from www.rarlabs.com. It has no restrictions and from what I have seen it lasts quite a while.
To reassemble the split files place them in the same directory, then right click on the first file in the series and choose an Extract option (winrar integrates into the shell by default, you can also turn that feature off if you like), winrar will find the rest of the files.