How to create text file.The filen name should be diffent then last created file.

Try this

string FileName=Test+DateTime.Now.ToString().Replace("/", "").Replace(".", "").Replace(":", "").Replace(" ", "")+".txt";
string text = "Test string to write inside the text file";
System.IO.File.WriteAllText(@FileName, text);

Concatenate file with FileName + HH:MM:SS
fileName024658

Try this

string path = @"D:\";
string fileName = "TestFile" + DateTime.Now.ToString().Replace("/", "").Replace(":", "").Replace(" ", "") + ".txt";
File.Create(path + fileName);

There are number of ways to solve this problem as:

1. You can use and variable appended with file name that will make it unique.

2. If you want to implement it with counter then you should save it in database or a file because when application will down all values of counter will automatically rest.

As suggested by Mehdi use Guid. Try as below code.

string myText = "Your Text";
string myFile = System.Guid.NewGuid().ToString() + ".txt";
System.IO.File.WriteAllText(myFile, myText);

Total concept of Guid is based on "Globally unique identifier".
http://en.wikipedia.org/wiki/Guid

Updated -
To read your saved text files, have a look at below links.
http://www.gridview.net/read-text-file-in-c-asp-net/
http://msdn.microsoft.com/en-us/library/ezwyzy7b.aspx
http://msdn.microsoft.com/en-us/library/aa287534(v=vs.71).aspx

Also don't forget Google is your Friend.
Google Search Result - To read text from File