As Arch4ngel wrote, you can build an array of words from the whole text using Split.
Now, you can use a Hashtable to count how many times each word appears in the text.
For example:
Hashtable htWords = new Hashtable();
String[] words;
...
words = txt.Split(); //txt is the string from the text file
Foreach (string word in words)
{
if (htWords.Contains(word))
htWords[word] = (int)htWords[word]+1;
else
htWords.add(word,1)
}
At the end the Hashtable should include a list of all the words and the number of times each word appears.