How can I insert a radical sign?

If that version of word does not have it available on the alternative character choices, then you will need to use a third party program. I use Foreigner and it works fine. And I see it is available here:
http://moorstation.org/typoasis/designers/DrBerlin/user.dtcc.edu/_berlin/font/utils.htm

HTH

go to start>programs>accessories>system tools>character map