Could I have a virus?

I have multiple user names on my computer but under one user name, I can't access my fire fox or internet explorer without a pop up saying this tread is harmful. there is a harmful code that can damage your computer. get protected with vista protection ( well something like that) It doesn't do this on any other user name. So what Im asking is... Is this effecting all users on this computer? Is some one trying to hack my computer or do i have a virus?