Windows Internet Security Blocks downloads from reputable publishers

I have just upgraded my HP Pavilion notebook to Windows 7. But when I try to download the iTunes program the Windows Internet Security block the process because of "Unknown publisher". I have tried through Tools>Internet Options to reduce the security level (Medium appears as low as it will go) but without success. How can I download this well known program? I have also had the same problem with the Uniblue Registry Booster that is recommended in this forum.

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