Put this in your .vimrc:

map <F5> :s.^//.. <CR> :noh <CR>
map <F6> :s.^.//. <CR> :noh <CR>

Now you use a visual line select (shift-V) to select what you want to comment, smack F5 and it's done. F6 uncomments.

As a bonus, use these to change indents. Change indent to your preference for indenting. I use 4 spaces. Use \t if you want a tab. Combined with shift-V, you can move whole blocks left and right easily.

map <F7> :s/^indent// <CR> :noh <CR> gv
map <F8> :s/^/indent/ <CR> :noh <CR> gv