Concerned I may have accidentally dropped malware (or something) onto my system First time poster in this part of Stack Exchange - apologies I have posted in the wrong section.

In short I have an iMac running 10.8. Most of the time I quite happily get about my business but about every 3-4 years I do something stupid. Today was one of those days.

Basically I was looking at an alternative method of running Mame (currently use the OpenEmu route). Basically downloaded SDLMame, then was going to go the QMC2 front end route for a GUI. Downloaded both, but when downloading QMC2 (from what is a legitimate site) - installed the package and it installed a folder into my Applications folder with no permission to access it. In short, I didn't like that at all (as that is a fairly non-standard thing), deleted the folder (securely) without installing the application file, and then ran Intego Virus Scanner with week old definitions, and ClamXAV. Neither picked anything up when downloading the packages (I always check all downloads that way) and neither picked anything odd up on my system afterwards (after deleting). In short my iMac hasn't behaved erratically since but I just wish to be 100% so is there anything else further than I can do? I have also run Etre Check, and it didn't pull anything obviously abnormal except for the fact that I need to update my Flash as there has been a new recommended version released etc.

I expect I may be having an over-reaction and it is possibly standard for that emulator (had never used it before) but I take the better to be safe than sorry approach.

Thanks in advance for any suggestions.
It looks like you did your due diligence and there is no evidence of a real malware problem - it sounds like their installer just screwed up the permissions.

Fire up Terminal, and do an ls -l of your Applications folder, looking for the QMC2 folder. I'd be willing to be it did something like installing as root (I bet the package asked for your password, right?) and missed correcting the permissions.

Once you've found it, you can do a chown (your username) -R (QMC2 path) to set everything to your user.