Here is the simplest way you can combine multiple text files into one single file.
Lets say you have four text files a.txt, b.txt, c.txt and d.txt and you want to merge the contents of all the text files into one target.txt.
To merge text files open the MS DOS command prompt and change the directory to where all files are stored. Now issue command : copy *.txt target.txt
This will merge all the text files in the directory and create a new file named target.txt