There are a couple of different things (well, more than a couple) that can possibly cause this type of scenario.
Some are malware related, some are down to the presence of an old (even factory installed but uninstalled) AV, some are down to firewall issues.
You say you've only ever used Avast on this machine. What did it come installed with, out of the shop, if anything?
Try downloading the free version of
MBAM (blue download button), install it, update it, run a quick scan. At completion of the scan a report will be produced. If malware is detected, select everything found and have MBAM remove it. If it prompts to reboot, do so promptly. Please post the scan report at the completion of this, whether malware is found or not.
Another possible answer/troubleshooting procedure is to download the Avast uninstall utility, and a completely new Avast installation file (get the uninstall utility
here ) Uninstall Avast from the control panel, reboot, run the uninstall utility, reboot, install afresh. Reboot when prompted.
If problems still remain after a full reinstall, it's likely malware or software confliction related. If malware is found by MBAM, that's likely to have been at least part of the problem, and, depending on the malware, might require further action. If there are still residual files from, say, Norton/Symantec present (possible even after an uninstall) a tool needs to be run to rid them from the machine.
Those are the most likely issues I know concerning this kind of problem.