What's in your .vimrc file?
nikhilpandit: I like autocomplete in my .vimrcfunction! InsertTabWrapper()
  let col = col('.')-1
  if !col || getline('.')[col-1]!~'\k'
    return "\<tab>"
  else
    return "\<C-P>"
  endif
endfunction
inoremap <tab> <C-R>=InsertTabWrapper()<CR>