Comment by Vendan

The very specific UAC one is secure, at least from anything that doesn't already have basically full control over your system, as it runs in the context of the SYSTEM account. The effect, and even much of the "alternate desktop", is trivial to reproduce, and is not as secure. One notable example is KeePass, which has an option to use a "Secure Desktop" for master password entry, but as it's done from the current user, is not secure against an attacker that understands what it's doing, though it will "bypass" a keylogger that's not designed to log "alternate desktop" interactions.