Editing txt files in hard drive c is problem? pls give me suggestion

Depends on where you're copying the text files.... if they are being copied from an encrypted source, yes, you will get that answer.
Secondly it is not advisable to copy files to the C drive base directory, or, C:\\.... Lots of viruses and other malware tend to do that so the system is built to try and battle such stuff thereby not giving you access to it.
You could try a work around by disabling administrative permissions using the control panel....UAC.... change user account control setting.