AVG Accelerator, being a part of excellent virus protection software, interferes with the download process and is reported to freeze Internet connection in some cases. To be able to download from YouTube, turn it off (AVG Antivirus will continue protecting your system).
- Open AVG Intertnet Security and go to Options → Advanced Settings → Web Browsing protection → Online Shield.
- Uncheck 'Enable AVG Accelerator' and press 'OK'.
Instruction for AVG 2012