Here is a sample console application that will accomplish this for you. You could get fancier and have it simply replace the existing file by opening the file and editing it in place if you want too.
Here is what TextFile1.txt looks like:
static void Main(string args)
StreamWriter sw = new StreamWriter("NewFile.txt");
using(StreamReader sr = new StreamReader("textfile1.txt",Encoding.ASCII))
line = sr.ReadLine();