I also had installation issue, both with v4.3 couple days ago and now same error with update v4.3.1
Both times downloaded the full installer for install on Windows 7. Both times entire directory "C:\Program Files\paint.net" was emptied clean of everything except for the staging folder which contains a .msi file. Double-clicking this file installs the app with no issue. So I'm still a happy camper - just passing this on in case there's something the developers need to address.