Hi all,
After investigation I found that it 'windows' WEBDAV limitation :
1) Alfresco webdav works fine with MS-OFFICE files (.doc.,.xls,.ppt).
i.e. using webdav feature of Alfresco we can online edit MS-OFFICE files
(.doc.,.xls,.ppt) without copying files on local system and then editing.
2) But other windows editor such as NOTEPAD and WORDPAD does not have inbuilt webdav-client support. Thus, files such as .txt,.rtf, etc. which are opened in these text editors are not online editable.