E505, File is write-protected

[Wolfgang Schmidt]

   Hi vimmers,
when I try to write (:w) a certain buffer, I get E505 (~ "file XXX is 
write-protected, override with w!").
The protection seems to be set by Vim, not the OS (WinXP). So I looked 
for the "readonly" option value,
but it's set to "noreadonly". I also checked "write", which is set to 
"write". Nevertheless, everytime I try to
:w the file, I get this E505 dialog. Any hints? I'm running Gvim 7.0 on 
Win XP)

Thanx in advance

   Wolfgang