1. I'm assuming you have no problems connecting to other websites, see if one of the antivirus software has (somehow?) flagged YouTube.
2. Disable all addons
3. Do a virus scan. Most virus scanners I've used have failed to find even a simple flashdrive virus I had, so you shouldn't always trust your AV.
4. If all else fails, try a different, less popular browser.
5. Reinstall Windows. I think sometimes viruses just screw up your system so badly and subtly that the only way to get rid of em is to reinstall windows and format your flash drives.
Reinstall Windows and it hasn't reappeared. Of course, you should only do this if all else fails and make sure to backup all important files.