This is happening:
In this case, it's happening with Microsoft Security Essentials, which is kind of a big deal for me. It fails to install without error if I hit cancel on the prompt.
This also happens basically any time I try to install something through Steam, but in those cases usually hitting Cancel lets the rest of the install go through without issue, probably because I already have the necessary files in the SxS folder.
Searching for this problem is hard. There is a ton of irrelevant bullshit and the people having the same problem as I am having never report success in fixing it. Anyone know what I should do?