Save XML tags and information to another file
There are some good points raised in comments about the requirements, and you really need to think about them in order to write code that solves the problem.  I can get you started with some feedback though.

Organize your code into a function, so that it's clear what the inputs and outputs are.

StringReader reader = new StringReader(text);
while ((text = reader.ReadLine()


This is a really odd reuse of the variable text.  Declare another string, maybe called line.  But also reconsider if a line-centric approach is even appropriate.  XML is pretty free about where whitespace (including newlines) can appear - what if there is a break between the tag name and some attributes?  How will this affect your solution?

For the Regex @"(&lt;.*&gt;)(.*)(&lt;\/.*&gt;)", note that all of these quantifiers are greedy, which may result in you capturing more than you want.  Of course, it may turn out with some clarified specs that a regex isn't even the appropriate tool to use here.