microsoft.public.windowsxp.security_admin

I am using Firefox for a web browser although IE is still installed.
When I am on the net surfing, IE suddenly opens and presents me with
an advertisement. If I wait a while without canceling the open IE
window, it will soon present me with another uncalled for ad.
Something is triggering this action. I've used all the spyware,
malware programs I can lay my hands on and nothing helps. I even ran
HiJakThis and nothing seemed to show up unusual. Can anyone help me?