Your idea should work and in terms of moss  technology you can try in below way.
1. Create a moss portal.
2. Create a custom  profile proerty to store and validate disclaimer.
3. Create a custom page  using Site Actions - > Create page option. Use blank webpart page. Make this as default page by changing link in welcome page which is in site settings under look and feel section.
4. Create a webpart for your disclaimar and on accept/reject button click update the users propfile disclaimer  custom property and programatically change
5. Create httpmodule or httpmodule or delegate control and inside any of those control check the disclaimer property and if user agreed then redirect to default.aspx page or else redirect the user to disclaimer custom apsx page.

I hope above steps should guide to make ur reqirement fulfill.

i have one doubt , in above steps if user try to acces the portal using hard coding url then how will get that discliamer page.

Do u mean user directly access any inner custom  pages inside portal? If yes please check below.

Generally httpmodule is executed on each and every page  request so any page that's under sitecollection will first execute httpmodule code and inside that code we will check current user disclaimer  property and if its already approved then it will render current requested page if not redirect back to disclaimr page.

DelegateControl is a control which is placed inside a master page. Generally any page in sharepoint will be inherited from master page so while creating any sites make sure ur master page has delegate control declared. So in delegate control u will mention ur logic of checking disclaimer property checking.