compare two strings or compare two files?

[bobbymusic]

In my form I have two rich text boxes.In both of them I put two different text files(.txt or .rtf).I looking for some code to compare them, and to have the result as marked differences into the second rich text box.How I have to compare them-as strings or as files? And how I can use mid$ to compare them letter by letter to have the right positions of differences?
Thanks in advance!!!