Replace 2 Occurances of Space (sed)

I have a large file that looks like the below output:

system     SUNWxwmod                        X Window System kernel modules
system     SUNWxwoft                        X Window System optional fonts
system     SUNWxwopt                        X Window System Optional Clients
system     SUNWxwpft                        X Window System Printer Fonts

I would like to add a ; after the first 2 occurances of space, for example the above would now look like:

system  ;  SUNWxwmod  ;                     X Window System kernel modules
system  ;  SUNWxwoft  ;                     X Window System optional fonts
system  ;  SUNWxwopt  ;                     X Window System Optional Clients
system  ;  SUNWxwpft  ;                     X Window System Printer Fonts

How can this be done with sed or can it?Thanks

I edited your snippets and rearranged the position of the semicolon so I hope this was correct - if not, you can edit it yourself of course. Please use code tags in future, ty.

$> sed 's/\([^ ]\)   /\1  ;/g' infile
system  ;  SUNWxwmod  ;                     X Window System kernel modules
system  ;  SUNWxwoft  ;                     X Window System optional fonts
system  ;  SUNWxwopt  ;                     X Window System Optional Clients
system  ;  SUNWxwpft  ;                     X Window System Printer Fonts