textwidth is fastened at 78, and "Function MyDiff already exists" message pops up when configuration update

Greetings,

I'm jaehak Kim and I downloaded Gvim for windows for the first time.
OS of computer is windows7 64bit.

It is very convenient , so I'm grateful to you.

but problem is generated, I wanted to edit the Gvim by _vimrc .
Some commands do not work.

when I command 'set tw=120' , it has no effect. Always textwidth is 78 in (*.txt), and 0 in (*.c)
editor is not compatible and nopaste , but I don't understand why this happening is happened

Would you tell me why it is happened?
and what should I do to change textwidth of gvim editor?


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plus, when I refresh configuration, by this command

map <F5> :source c:\Program Files <x86>\Vim\_gvimrc<CR>

message like "Function MyDiff already exists~~" was shown.

There is no inconvenience in use, but it is just on my mind.

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