[h-e-w] emacs grep on Windows

[Kenneth Goldman]

I use igrep, but it seems to be broken on Windows with emacs 23.  I'd like 
to switch to grep.

The manual says:

        If you specify a prefix argument for M-x grep, it finds the tag 
...

There's no sample code for how to actually set the prefix argument.  I 
know I can type C-u every time, but how do I assign it to a key 
(global-set-key)?