Win10 gvim7.4 seemly not have enough permission(default install)

I install gvim7.4 in the default directory 'C:\Program Files (x86)\Vim' on
windows10 x64 system.
I open the origin _vimrc file, and run
:vert diffsplit _vimrc_my
It shows error:
E810: Cannot read or write temp files
E97: 无法创建diff
I open the origin _vimrc file again and delete everything, and run
:vert diffsplit _vimrc_my
OK.

Question is:
Where the temp files save when do diff using the origin _vimrc config?
Why this will hapen on win10? my win7 system is OK.

========== the origin _vimrc file contents
"set nocompatible
"source $VIMRUNTIME/vimrc_example.vim
"source $VIMRUNTIME/mswin.vim
"behave mswin
"
"set diffexpr=MyDiff()
"function MyDiff()
" let opt = '-a --binary '
" if &diffopt =~ 'icase' | let opt = opt . '-i ' | endif
" if &diffopt =~ 'iwhite' | let opt = opt . '-b ' | endif
" let arg1 = v:fname_in
" if arg1 =~ ' ' | let arg1 = '"' . arg1 . '"' | endif
" let arg2 = v:fname_new
" if arg2 =~ ' ' | let arg2 = '"' . arg2 . '"' | endif
" let arg3 = v:fname_out
" if arg3 =~ ' ' | let arg3 = '"' . arg3 . '"' | endif
" let eq = ''
" if $VIMRUNTIME =~ ' '
" if &sh =~ '\<cmd'
" let cmd = '""' . $VIMRUNTIME . '\diff"'
" let eq = '"'
" else
" let cmd = substitute($VIMRUNTIME, ' ', '" ', '') . '\diff"'
" endif
" else
" let cmd = $VIMRUNTIME . '\diff'
" endif
" silent execute '!' . cmd . ' ' . opt . arg1 . ' ' . arg2 . ' > ' . arg3 . eq
"endfunction
========== the origin _vimrc file contents

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