What avast program are you running avast Free/Pro/IS/Premium ?
The HTTPS scanning is only available on XP SP3 and this is a bit of anomaly as the 64bit version of XP Pro doesn't have an SP3 as the OS is based on windows 2003.
So it shouldn't be trying to scan HTTPS content.
As for issues with HTTP pages that is strange, but after a program update some elements of avast (commonly Web Shield access) could be blocked by your firewall.
What is your firewall and does it allow avastSvc.exe internet access ?
Does your Web Shield Settings look like my attached image ?
You could try an avast Repair as a start point, Control Panel > Add remove programs > avast! > Change - scroll down to Repair and OK your way out, reboot.