SteamSetup.exe - Access is denied

[SystemParadox]

I've got a laptop here running Windows 7 that previously had steam installed. Due to some issues it was uninstalled and I am now trying to reinstall it. However, there is something very strange going on with the steam installer.

Double-clicking the icon gives the error "Windows cannot access the specified device, path, or file. You may not have the appropriate permissions to access the item". The same thing happens if I right-click and try "Run as administrator".

Running it from a command prompt (or an administrator command prompt) just prints "Access is denied".

I have tried sfc /scannow, but that did not report any issues.

One thing I have noticed is that Windows is not displaying the steam icon for the installer exe. Nor does it display any details such as the publisher or version number (which do show correctly on other machines). Also, it takes forever to open the properties window.

It originally felt like a corrupted file, but I have tried clearing the Chrome cache and redownloading multiple times and the issue persists. I have also tried copying a known-working version from another machine and exactly the same thing happens.

My best guess at the moment is that there is a library corrupted somewhere, but I cannot find out what it is. Does anyone know what libraries SteamSetup.exe depends on?

I have tried lots of other programs and they all seem to be working fine. So far the steam installer is the only one affected.

Any suggestions would be much appreciated.

Thanks.

[ReBoot]

https://support.microsoft.com/en-us/help/929135/how-to-perform-a-clean-boot-in-windows
Clean boot and it should work. This problem typically means that some dumb program made by dumb people that's installed on your computer prevents Steam from working.

[SystemParadox]

Good suggestion. Unfortunately even a minimal boot with no services doesn't seem to have helped.

[SystemParadox]

Aha. Safe mode seems to fix it. I have managed to get steam installed. I wonder how I can narrow this down to find out what the issue is. If diagnostic startup doesn't fix it, but safe mode does, what might that imply? What runs in diagnostic startup that doesn't run in safe mode?

[ReBoot]

https://support.steampowered.com/kb_article.php?ref=9828-SFLZ-9289&l=
That's what it implies.

[SystemParadox]

This turned out to be an issue with antivirus software silently blocking the installer exe. I believe AVG was to blame (although it may have been Microsoft Security Essentials).