Don’t worry about press-this, it’s a core file that is safe, it’s just shows up in the Potential Threats because it uses an eval method that some hacker use to hide their malicious code. I have not added it to my global whitelist for WP 3.9 yet but you can whitelist it.
As for the Complete Scan “freezing”, there could be a number of reasons for this. Did the similar forum topic not an answer that worked for you?
If it is staying on 0% and endlessly “Preparing” directories but the directories keep changing then you probably have a recursive symlink in your scan path. The easy work-around for this is to set your Scan Depth to a positive number (like 6 or 8).
If the directories that it is preparing do not change than it is getting stuck on something. If it’s always getting stuck on the same directory you could try adding that one to the list of folders to skip to see if it will go on.