jonv -- regarding keyboard error when you sked the boot scan...
this happened to me when I first tried to install Avast! home edition. The solution I found was to completely uninstall Avast!, delete all Avast! files from your program files, reboot, run disk cleanup, defrag, reboot,
go to start>my computer>right click on C drive>tools and schedule CHKDSK (making sure you check both options). Okay your way out and shut down your computer (do not restart). Wait one minute. Turn on computer and CHKDSK should run after a 10 second countdown on the blue screen. After computer reboots, then download Avast!, install and boot scan should be okay.
If CHKDSK doesn't work, or the Avast! boot scan reads keyboard error again, I found it's one of two things -- either something leftover from another antivirus or, as was the case in my situation, a conflict with one of the Windows Updates. That was cured by an XP reformat, reinstall, CHKDSK and then Avast! install and all's been well. Hope this helps.
Dave