In my case and while installing VS 2015 on Windows7 64x SP1, I experienced the same so tried to cancel and download/install the KBKB2999226 separately and for some reason the standalone update installer also get stuck searching for updates.
Here what I did:
- When the VS installer stuck at the KB2999226 update I clicked cancel.
- Installer took me back to confirm cancellation, waited for a while then opened the windows task manager and ended the process of wuse.exe (windows standalone update installer)
- On the VS installer clicked "No" to return to installation process. The process was completed without errors.