HJsplit

Download HJsplit from here.

After unraring the file, open the hjsplit app, click on the join tab.

It in the new window select input file, browse to the place where you downloaded the file and select the file with extension 001

example :  The.Mechanic.mkv.001

after that click join at the bottom. Tada your file will be at the same location as the 001 file.