Vista - files blocked if html or htm

I have Vista Busniess and am domain and computer administrator an this laptop.
I have some file located e.g. in D:\Folder\Folder2\file.txt which I can
easily edit, rename etc... I have full control of this file. Once I close the
file, rename into file.html and open it then with an editor I can't save
changes anymore, it says like "File/Device may be read only or open for write
by another application". Neither if this is true. Renaming the file back to
file.txt allows me to change contetn again, also I can rename the file.html
into file1.html, this is possible, just editing the content is not
allowed.This is true for any html file not just for one.
I have to mention that UAC is turned of on this PC.

Kind regards,
Dieter