USB Blaster with Windows 8?

No, I didn't mean to say that your drivers are no longer signed only that the policy for loading drivers has changed.

The driver itself and its signature have not changed.

When Windows 8 is installed over an existing OS installation, it will often ignore the existing drivers. If these existing drivers are not signed or are changed, it will not allow you to install them even if you manually attempt to install them.

For example, my sound drivers that came with my notebook are signed and the installer is signed but when I over-installed Win 8 RTM they were replaced by the class driver.

Another example, I'd like to test a pre-release driver package that I've self signed. The installer (or manual update) will attempt to install and load this driver but I'll get a pop-up dialog box with a warning. If I modify any of the files in the driver package after signing then I'll get an error.

I believe, based on the error you posted, that your driver package was modified after signing. For example, Altera may have taken the signed driver package from FTDI, changed file names and configuration files, combined with other files to create the installer package and signed the installer package but didn't properly sign the modified drivers. Everything installed fine using the signed installer under your previous OS but the drivers were not re-installed when you upgraded to Win8. Attempting to update the driver under device manager will generate the type of error message you describe. You may be able to re-install your current version using the Altera Quartus installer but not knowing the exact cause of your error I can't say for sure if this will be successful.

I sincerely hope that I have not made the situation more confusing for you.