I've read through all these posts concerning this issue. The same issue came up on a Dell Dimension E520 with Windows Vista. (Checking it for a family friend) The PC had Avast Free 5 and was recently upgraded to 6, but the upgrade was done without uninstalling Avast 5 manually or first. The upgrade did not go well, so I uninstalled Avast, then ran their cleaner and then installed Avast 6 again. The install went fine, but a Boot Scan will stop at 5% and on either a .DOC, .XML, .PDF type file. We stop the Boot Scan and try to scan the folder these files are in and the scan will not run. Scanning other folders with Avast 6 works fine though. We've deleted some of the files the scan will appear to stop or pause on and there's no difference in results; The scan will still just hang on a .DOC, .PDF, etc.
The PC has the Vista 32 Bit OS, 1 GB Ram, 1.87 or so Processor. Also running AdAware, MalWare Bytes and Windows Defender. There WERE NO Issues until the upgrade to Avast 6 took place. How can someone say it's something else when the issue was not an issue until Avast 6?