set pastetoggle=<Esc>[201~

noremap <Esc>[200~ :set paste<cr>0i

inoremap <Esc>[200~ <c-o>:set paste<cr>

cnoremap <special> <Esc>[200~ <nop>

cnoremap <special> <Esc>[201~ <nop>

let &t_ti="\<esc>[?2004h"

let &t_te="\<esc>[?2004l"

the ti/te sequences are passed from Vim to the client which tells Putty (and others) to bracket all pasting with \e[200~ and \e[201~. The other commands handle these sequences to make pasting happen transparently without having to hit F2 ;o)