Right-click the file and uncheck "Read only".
If it doesn't work, right-click, "security" tab and see if you have permission to read, write, modify, execute, etc. If some of these is unchecked, use the edit button, add your username, check in everything and then press ok.
(My OS is portuguese, so there might be some variation with the translated words above)
(Also, I'm using Vista for a long time, so there might be some variations to the steps mentioned above
)