I've used Total Uninstall for years and highly recommend it. It's true to its name and when you uninstall software, it cleans up as if the software was never installed. No traces are left behind. It will even decreases the shared file counters when uninstalling if you set it up that way.
http://www.avira.com/en/avira-free-antivirus ( Antivirus software , free )
http://www.martau.com/uninstaller-download.php ( Total uninstall )
That's what I always install first after formatting my hard drive.
Well this will be a PITA but I think it should work.
Install Total Uninstall on another computer with the same operating system. It might work OK on a different version of Windows, but your best bet is to get it as close as you can. Close all programs. I mean everything you possibly can, including all the crap in the system tray. Do a monitored install and install the game. TU will scan the registry and file system before the install, then you install, and it will scan again after and tell you all the changes to the registry and all the files that were added/altered. I believe there is an export option and you can save that report, otherwise you'll just have to view it in TU. You'll see the registry tree and file tree which will show you what entries you need to revert and what files/folders you need to delete.
there might be, one more thing you can do.
this program is called Total Uninstall by Maratu i suggest downloading it, and running it. It will scan the whole computer for left over files, programs (in its full use) and so forth. It'll install a 30day trial, and if you buy it- you buy it. so just, use the trial, and see if any left-over files for roblox are there.
I'm not completely sure, but it sounds like Total Uninstall should be able to take care of the possible negative effects.
edit: Forgot this is paid software, lol. It analyzes your system before installing a new program and creates a restore point. I'm not sure if there are free alternatives out there, link 'em if you know any better options.