Redirect non-www to www but NOT https URLs

I want to redirect http://site.com to http://www.site.com but I want to leave any https://site.com/securepage.htm URLs alone. Or redirect http://site.com to http://www.site.com AND https://site.com/securepage.htm to https://www.site.com/securepage.htm

I've found a few pieces of code and nothing has worked. What I end up with is code that redirects https://site.com/securepage.htm to http://www.site.com/securepage.htm and I loose the security.

Any help would be great.

Thanks