Is the current www.abc.com website based on Sharepoint?
If not, the easiest way is to create a different Sharepoint/web farm on which you install Sharepoint. That way the current webserver can still host the www.abc.com and the new Sharepoint farm can create a site with the hostheader www.abc.com also.
As long as your internal DNS or hosts file(s) don't mess things up you'll be able to do this.
If the current site is based on Sharepoint, maybe the easiest solution is to create the site on a different port, so www.abc.com:81.
I'm not much of an ISA/Forefront expert, but I think you can set those up to redirect a request to the proper site:port.
Other solution might be to just 'Stop' the current website in IIS. That way a new website will be created with the hostheader. Just remember to backup your current virtual directory and other stuff you've configured for the current website.
Also Nick Hadlee his suggestion is a good solution to your problem.