Other Help Topics :: Saving files under MP

Hi,
When I create a file under MP, I can edit, save this file. Everything is going fine. But when I open an existing file that I downloaded (for instance: soft source files...) and I try to modify it, I can't save it!
MP gives me: 'can't create file blablabla...'

I checked the rights on those files. The access is read and write for normal users and root.
Do you have any suggestion?
Cheers
Bill_oute