My first step would be to contact your ISP and tell them about the invalid or expired signature problem you are experiencing. It is possible that they have a newer download OR that they need to update something in relation to the download.
A second possibility is that your computer clock needs to be reset. Perhaps it thinks it is a later year than it actually is. That would also cause the problem you have. If you need assistance in checking or changing it, please let us know and also what Operating System you use: Windows XP, Vista etc.
If neither of those are the issue, then I believe the answer lies in how your Internet Options handles security certificates. However, I'm not too familiar with that aspect as I mostly don't use Internet Explorer. It would be helpful to know which version of Internet Explorer you have: IE6, IE7, etc.
Edited by Orange Blossom, 09 June 2008 - 09:25 PM.
Delete extra signature. ~ OB