Try putting following lines in .vimrc
" Make shift-insert work like in Xterm
map <S-Insert> <MiddleMouse>
map! <S-Insert> <MiddleMouse>
I had the same problem and after putting this lines in .vimrc shift insert works like a charm.
Refrence:http://tech.groups.yahoo.com/group/vim/message/104539
Best Answer
In the command line mode type below line and press Enter
And if you want to make it permanent then write that line (set noshowmode) without colon inside .vimrc file inside your home directory