are you usign active directory? this is two part...
if you are, you implemet a domain policy. the policy is under user cofiguration, Windows settings, internet explorer maintenece, URL, Important URL's. Then specify the Homepage URL. Apply the doimain policy and you are done.
If you don't use AD, you can do this all though NT4 policy, but that is not as easy. Also, you can do it though registry structure. Are these users or admins/power users?