It could have very-well been something else that was the culprit, but after using online Youtube to MP3 converters on a regular basis, I had to reinstall the OS on both my machine and my wife's after a while, as they both got extremely slow and sometimes unresponsive to typing and mouse movements.
I ran spyware/malware scans, virus scans, and even defragged the hard drives and cleaned the registries on both machines to no avail.
I say all of that to ask if it's possible that these online conversion tools could cause damage to your computer or not?