It may be possible that you downloaded the Creative Cloud trial in error (the perpetual / stand alone license will not work on this version). Uninstall the existing version using your operating systems uninstall process, this will not interfere with your catalog, preferences or personal files.
Download the latest LR stand alone version from the link below and install. Insert your upgrade key and it should accept it just fine.
I will try, but I don't think this will work. I am in the process of downloading from your link, but the file appears to be identical to the one I had already downloaded. I cannot uninstall the previous version because it is already uninstalled, done when I was in the process of upgrading from 5.6 to 5.7. The program no longer appears as installed in control panel and there are no files shown in the relevant Adobe program folder. I'll confirm my tentative conclusion after the file from your downlink has finished downloading and I've tried to install it, but that is my expectation.
I was not advising to remove the earlier version but the 5.7 update version you installed. And yes the both versions are the same software but they are licensed differently. The perpetual version require the input of the license key manually and the Creative Cloud version is done through the Adobe Creative Cloud Application Manager.
Didn't work. Same problem. Just to clarify. The files are identical - not just the same software, but identical files - perhaps you linked me to the cc version by mistake - I can't find on the Adobe website a reference to a standalone version of the file. Also, just to be clear, when trying to install version 5.7 it never gets to installing anything. It aborts early on when trying to "validate". So there is nothing to remove. And the first time I tried it, it removed my prior version 5.6 so I now have nothing. Thanks for your help. I appreciate it.
Have you tried right clicking on the download .exe file and choosing the option to "Run as Administrator" ?
Under win 7 it should first unpack the contents of the file into a folder on your desktop and then the installation procedure should continue. It may invoke a popup asking for your password to continue.
At what stage does it abort the install? see screen capture.
From Googling the error message, it may be a specific Windows update that causes the problem: KB2918614 <http://support.microsoft.com/kb/2918614/>
Go to Control Panel / Programs and Features / View Installed Updates then find the update / right-click / Uninstall.
If you sort by name and look in the Microsoft Windows area the updates will be divided into a few groups with the numbers in order in each group.