This seems to be the same problem you were having unintalling ST5.
Try cleaning out C:\Program Files (x86)\Common Files\InstallShield\Professional\RunTime. Then rerun the SE installer.
Also, see if this helps :
I warned you of this in your ST5 uninstall post, and predicted that you would have this same issue when trying to install ST8. This vbscript error is normally indicative of a wider system issue and is not specifically Solid Edge related.
You can attempt to spend many hours trying to resolve this issue or you can address it up front and simply rebuild your system. Having experienced these types of issues many times, I personally would just rebuild the system -- it will be faster in the long run.
I hate to say it, but I agree with @uk_dave. I talked to our developers, and they agreed that an error in the VBScript.dll implies that something is wrong with your operating system - it's not something that SE can resolve. It might be possible to do a repair on your OS installation rather than a full rebuild, but that might be the most efficient way to get back to a stable system.
Sorry for the bad news...