How to convert .hlp files to .chm file in windows 7 ???

Try opening the RoboHelp HTML application (not the RoboHelp for Word application used to create the .HLP file). There create a new project specifying to import the HLP file. From memory that does a pretty good job of bringing across the content. Once there it is easy to create a CHM file. If you need to know more specifics, you'll need to tell us which version of RoboHelp you have.