>>>For writing to a file it can be opened with FOPEN() in WriteOnly mode and Read/Write mode.
>>>
>>>Is there a performance issue when opening it Read/Write?
>>
>>Most likely not. Only thing I can think of that MIGHT slow it down would be if the file was really large and your anti-virus software took a long time to scan it.
>
>To scan it? If I try to access the file from the Explorer (in my case Total Commander), just for simple reading, then there is an Access Denied, no matter whether it is WriteOnly or Write/Read. Does a virus scanner have access where I do not have?

Is the file locked by something? You can use this tool to find out..
http://www.emptyloop.com/unlocker/

Also you can use this tool to tweak the file attributes...
http://download.cnet.com/Attribute-Changer/3000-2248_4-13676.html